Syncing docs: Japanese translation updates

h/t Shin-ichi

docs/ja_JP.UTF-8/EXTENDING.txt

@@ -15,7 +15,7 @@
                   |       * Personal Files                   |
                   |  * Brushes                               |
                   |       * Brush Options                    |
-                  |  * Stamps                                |
+                  |  * はんこ                                   |
                   |       * Stamp Images                     |
                   |       * Stamp Descriptive Text           |
                   |       * Stamp Sound Effects              |
@@ -232,7 +232,7 @@ Brush Options
    it's because you forgot to use alpha transparency! See the 'PNG
    documentation' in Tux Paint for more information and tips.
 
-                                     Stamps
+                                      はんこ
 
    All stamp-related files go in the "stamps" directory. It's useful to
    create subdirectories and sub-subdirectories there to organize the stamps.
@@ -642,7 +642,7 @@ Scene-Style Starters
    "msgstr" lines directly below the corresponding "msgid" lines. (Note: Do
    not remove the quotes.)
 
-   Example:
+   例:
 
      msgid "Smudge"
      msgstr "Manchar"
@@ -713,7 +713,7 @@ Scene-Style Starters
    Start additional character mapping sections with a line containign the
    word "section".
 
-   Example:
+   例:
 
      # Hiragana
      304B ka -