Mention Japanese change in documentation

doc/emacs/mule.texi

@@ -397,7 +397,7 @@ messages.  But if your locale matches an entry in the variable
 coding system instead.  For example, if the locale @samp{ja_JP.PCK}
 matches @code{japanese-shift-jis} in
 @code{locale-preferred-coding-systems}, Emacs uses that encoding even
-though it might normally use @code{japanese-iso-8bit}.
+though it might normally use @code{utf-8}.
 
   You can override the language environment chosen at startup with
 explicit use of the command @code{set-language-environment}, or with

etc/NEWS

@@ -217,6 +217,11 @@ regular expression was previously invalid, but is now accepted:
 ---
 ** New input methods 'hawaiian-postfix' and 'hawaiian-prefix'.
 
++++
+** In Japanese environments that do not specify encodings and are not
+based on MS-Windows, the default encoding is now utf-8 instead of
+japanese-iso-8bit.
+
 +++
 ** New function 'exec-path'.
 This function by default returns the value of the corresponding

lisp/language/japan-util.el

@@ -29,11 +29,9 @@
 
 ;;;###autoload
 (defun setup-japanese-environment-internal ()
-  ;; By default, we use 'iso-2022-jp for default coding system.  But, the
-  ;; following prefer-coding-system will override it.
-  (if (memq system-type '(windows-nt ms-dos cygwin))
-      (prefer-coding-system 'japanese-shift-jis)
-    (prefer-coding-system 'utf-8))
+  (prefer-coding-system (if (memq system-type '(windows-nt ms-dos cygwin))
+			    'japanese-shift-jis
+			  'utf-8))
   (use-cjk-char-width-table 'ja_JP))
 
 (defconst japanese-kana-table