Wrapped up EXTENDING re: on-screen keyboard

I did my best. :)

docs/en/EXTENDING.txt

@@ -754,14 +754,13 @@ On-screen Keyboard
      be shared by different layouts). We'll use the QWERTY keyboard as an
      example:
 
-  qwerty.layout
+  Layout overview file ("qwerty.layout")
 
        This is a text file that specifies the other files used to describe
        the layout and key mappings.
 
          layout qwerty.h_layout
          keymap us-intl-altgr-dead-keys.keymap
-         dead_keys_map dead_keys.map
          composemap en_US.UTF-8_Compose
          keysymdefs keysymdef.h
          keyboardlist qwerty.layout default.layout
@@ -770,7 +769,10 @@ On-screen Keyboard
        any text following a "#" (pound/hash) character — it can be used to
        denote comments, as seen in the example above.
 
-  qwerty.h_layout
+       The "keyboardlist" line describes which layouts to switch to, when the
+       user clicks the left and right buttons on the keyboard. (See below.)
+
+  Keyboard layout file ("qwerty.h_layout")
 
        This describes how big the keyboard is (as a W×H grid), and lists each
        key with its numeric keycode (see the "keymap" file, below), the width
@@ -827,7 +829,14 @@ On-screen Keyboard
        [CapsLock], while numeric keys ([1], [2], etc.), [Space], and so on,
        will not.
 
-  us-intl-altgr-dead-keys.keymap
+       Keycodes up to "8" are reserved for internal use. The ones currently
+       used are described below.
+
+         * 0 — empty button
+         * 1 — next layout (per the layout file's "keyboardlist" setting)
+         * 2 — previous layout (per the layout file's "keyboardlist" setting)
+
+  Keymap file ("us-intl-altgr-dead-keys.keymap")
 
        This file defines which numeric keycodes (seen in the keyboard layout
        files, such as "qwerty.h_layout" described above) should be mapped to
@@ -869,6 +878,24 @@ On-screen Keyboard
          keycode 62 = Shift_R NoSymbol Shift_R Shift_R
          ...
 
-     ...Work in progress...
+  Composemap file ("en_US.UTF-8_Compose")
+
+       This file describes characters that can be composed by multiple
+       inputs. For example, "[Compose]" followed by "[A]" and "[E]" can be
+       used to create the "æ" character.
+
+       The file that comes with Tux Paint is based on the US English UTF-8
+       (Unicode) composemap that comes with X.Org's X Window system. The
+       current version from the Xlib library as a web page at
+       https://www.x.org/releases/current/doc/libX11/i18n/compose/en_US.UTF-8.html).
+
+  Keysym definitions file ("keysymdef.h")
+
+       This file (which is a C programming language header file) is also from
+       the X Window System. It defines the Unicode values of each keycap
+       (e.g., "XK_equal" corresponds to "U+003D", for the character "="
+       ("EQUALS SIGN").
+
+       It is unlikely that any modification will be required of this file.
 
      ----------------------------------------------------------------------

docs/en/html/EXTENDING.html

@@ -1594,7 +1594,7 @@
         the QWERTY keyboard as an example:
       </p>
 
-      <h3><code>qwerty.layout</code></h3>
+      <h3>Layout overview file ("<code>qwerty.layout")</code></h3>
       <blockquote>
         <p>
           This is a text file that specifies the other files used to describe the
@@ -1605,7 +1605,7 @@
             <code>
               layout qwerty.h_layout<br/>
               keymap us-intl-altgr-dead-keys.keymap<br/>
-              dead_keys_map dead_keys.map<br/>
+              <!-- dead_keys_map dead_keys.map --><!-- br/ -->
               composemap en_US.UTF-8_Compose<br/>
               keysymdefs keysymdef.h<br/>
               keyboardlist qwerty.layout default.layout
@@ -1618,9 +1618,15 @@
           "<code>#</code>" (pound/hash) character — it can be used to
           denote comments, as seen in the example above.
         </p>
+        <p>
+          The "<code>keyboardlist</code>" line describes which
+          layouts to switch to, when the user clicks the
+          left and right buttons on the keyboard.
+          (See below.)
+        </p>
       </blockquote>
 
-      <h3><code>qwerty.h_layout</code></h3>
+      <h3>Keyboard layout file ("<code>qwerty.h_layout</code>")</h3>
       <blockquote>
         <p>
           This describes how big the keyboard is (as a W&times;H grid),
@@ -1689,9 +1695,26 @@
           (<b>[1]</b>, <b>[2]</b>, etc.), <b>[Space]</b>, and so on,
           will not.
         </p>
+        <p>
+          Keycodes up to "<code>8</code>" are reserved for internal use.
+          The ones currently used are described below.
+        </p>
+        <ul>
+          <li>
+            <code>0</code> &mdash; empty button
+          </li>
+          <li>
+            <code>1</code> &mdash; next layout
+            (per the layout file's "<code>keyboardlist</code>" setting)
+          </li>
+          <li>
+            <code>2</code> &mdash; previous layout
+            (per the layout file's "<code>keyboardlist</code>" setting)
+          </li>
+        </ul>
       </blockquote>
 
-      <h3><code>us-intl-altgr-dead-keys.keymap</code></h3>
+      <h3>Keymap file ("<code>us-intl-altgr-dead-keys.keymap</code>")</h3>
       <blockquote>
         <p>
           This file defines which numeric keycodes (seen in the
@@ -1734,10 +1757,37 @@
         </blockquote>
       </blockquote>
 
+      <h3>Composemap file ("<code>en_US.UTF-8_Compose</code>")</h3>
+      <blockquote>
         <p>
-        ...Work in progress...
+          This file describes characters that can be composed by multiple
+          inputs. For example, "<b>[Compose]</b>" followed by "<b>[A]</b>" and
+          "<b>[E]</b>" can be used to create the "<code>æ</code>" character.
         </p>
+        <p>
+          The file that comes with <cite span="white-space: nowrap;">Tux Paint</cite>
+          is based on the US English UTF-8 (Unicode) composemap that
+          comes with X.Org's <cite style="white-space: nowrap;">X Window system</cite>.
+          The current version from the <cite>Xlib</cite> library as a web page at
+          <a href="https://www.x.org/releases/current/doc/libX11/i18n/compose/en_US.UTF-8.html">https://www.x.org/releases/current/doc/libX11/i18n/compose/en_US.UTF-8.html</a>).
+      </blockquote>
 
+      <h3>Keysym definitions file ("<code>keysymdef.h</code>")</h3>
+      <blockquote>
+        <p>
+          This file (which is a <cite>C</cite> programming language
+          header file) is also from the
+          <cite style="white-space: nowrap;">X Window System</cite>.
+          It defines the Unicode values of each keycap
+          (e.g., "<code>XK_equal</code>" corresponds to
+          "<code>U+003D</code>", for the character "<code>=</code>"
+          ("<code style="white-space: nowrap;">EQUALS SIGN</code>").
+        </p>
+        <p>
+          It is unlikely that any modification will be required of
+          this file.
+        </p>
+      </blockquote>
     </blockquote>
 
     <hr size="2" noshade>