Doc'ing buttonsize in OPTIONS

docs/en/html/OPTIONS.html

@@ -32,7 +32,7 @@
       </p>
 
       <p>
-        December 27, 2020
+        January 18, 2021
       </p>
     </center>
 
@@ -261,6 +261,26 @@
           </p>
         </dd>
 
+        <dt>
+          <code><b>buttonsize=<i>SIZE</i></b></code>
+        </dt>
+
+        <dd>
+          <p>
+            Set the pixel size of buttons in <i>Tux&nbsp;Paint's</i>
+            user interface (overriding the default of "48").
+            Useful when using very high-density displays or coarse input
+            devices, such as eye gaze trackers.
+          </p>
+
+          <p>
+            The <i>SIZE</i> value should be presented in pixels,
+            and can be anything between 24 and 192, inclusive.
+            Most buttons are square, and this will affect their
+            width and height.
+          </p>
+        </dd>
+
         <dt>
           <code><b>allowscreensaver=yes</b></code>
         </dt>