Improve the documentation of prefer-utf-8 and related issues.

 lisp/international/mule.el (define-coding-system): Doc fix.

 etc/NEWS: Document prefer-utf-8 coding-system and the new attributes
 :inhibit-null-byte-detection, :inhibit-iso-escape-detection, and
 :prefer-utf-8.

4 files changed, 49 insertions, 12 deletions

diff --git a/etc/ChangeLog b/etc/ChangeLog
index 84c5941e33f..02b13e12945 100644
--- a/etc/ChangeLog
+++ b/etc/ChangeLog
@@ -1,3 +1,9 @@
+2013-07-13  Eli Zaretskii  <eliz@gnu.org>
+
+        * NEWS: Document prefer-utf-8 and the new attributes
+        :inhibit-null-byte-detection, :inhibit-iso-escape-detection, and
+        :prefer-utf-8.
+
 2013-07-13  Leo Liu  <sdl.web@gmail.com>
 
         * NEWS: Mention new value for ido-use-virtual-buffers.
diff --git a/etc/NEWS b/etc/NEWS
index c3be0dfb9d4..03174a0137f 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -599,6 +599,35 @@ Emacs uses `image-default-frame-delay'.
 *** New functions `image-current-frame' and `image-show-frame' for getting
 and setting the current frame of a multi-frame image.
 
+** Changes in encoding and decoding of text
+
+---
+*** New coding-system `prefer-utf-8'.
+This is like `undecided' but prefers UTF-8 on decoding if the text to
+be decoded does not contain any invalid UTF-8 sequences.  On encoding,
+any non-ASCII characters are automatically encoded as UTF-8.
+
+---
+*** New attributes of coding-systems whose type is `undecided'.
+Two new attributes, `:inhibit-null-byte-detection' and
+`:inhibit-iso-escape-detection', determine how to detect encoding of
+text that includes null bytes and ISO-2022 escape sequences,
+respectively.  Each of these attributes can be either nil, zero, or
+t.  If it is t, decoding text ignores null bytes and, respectively,
+ISO-2022 sequences.  If it is nil, null bytes cause text to be decoded
+with no-conversion and ISO-2022 sequences cause Emacs to assume the
+text is encoded in one of the ISO-2022 encodings, such as
+iso-2022-7bit.  If the value is zero, Emacs consults the variables
+inhibit-null-byte-detection and inhibit-iso-escape-detection, which
+see.
+The new attribute `:prefer-utf-8', if non-nil, causes Emacs to prefer
+UTF-8 encoding and decoding, whenever possible.
+
+These attributes are only meaningful for coding-systems of type
+`undecided'.  (The type of a coding-system is determined by its
+`:coding-type' attribute and can be accessed by calling the
+`coding-system-type' function.)
+
 ** The function `set-visited-file-modtime' now accepts a 0 or -1 argument
 with the same interpretation as the returned value of `visited-file-modtime'.
 
diff --git a/lisp/ChangeLog b/lisp/ChangeLog
index 4bfc800913a..1c01afd8576 100644
--- a/lisp/ChangeLog
+++ b/lisp/ChangeLog
@@ -1,5 +1,7 @@
 2013-07-13  Eli Zaretskii  <eliz@gnu.org>
 
+        * international/mule.el (define-coding-system): Doc fix.
+
         * simple.el (default-font-height): Don't call font-info if the
         frame's default font didn't change since the frame was created.
         (Bug#14838)
diff --git a/lisp/international/mule.el b/lisp/international/mule.el
index 3577e0e9152..28542835a5f 100644
--- a/lisp/international/mule.el
+++ b/lisp/international/mule.el
@@ -672,7 +672,7 @@ is unsuitable for the top-level media type \"text\".
 
 VALUE must be a list of symbols that control the ISO-2022 converter.
 Each must be a member of the list `coding-system-iso-2022-flags'
-\(which see).  This attribute has a meaning only when `:coding-type'
+\(which see).  This attribute is meaningful only when `:coding-type'
 is `iso-2022'.
 
 `:designation'
@@ -692,7 +692,7 @@ to GN.  If the list contains 96, any charsets whose whose ranges are
 96 long can be designated to GN.  If the first element is a charset,
 that charset is initially designated to GN.
 
-This attribute has a meaning only when `:coding-type' is `iso-2022'.
+This attribute is meaningful only when `:coding-type' is `iso-2022'.
 
 `:bom'
 
@@ -712,7 +712,7 @@ are 0xFF 0xFE, use the cdr part coding system of the value.
 Otherwise, treat them as bytes for a normal character.  On encoding,
 produce BOM bytes according to the value of `:endian'.
 
-This attribute has a meaning only when `:coding-type' is `utf-16' or
+This attribute is meaningful only when `:coding-type' is `utf-16' or
 `utf-8'.
 
 `:endian'
@@ -720,37 +720,37 @@ This attribute has a meaning only when `:coding-type' is `utf-16' or
 VALUE must be `big' or `little' specifying big-endian and
 little-endian respectively.  The default value is `big'.
 
-This attribute has a meaning only when `:coding-type' is `utf-16'.
+This attribute is meaningful only when `:coding-type' is `utf-16'.
 
 `:ccl-decoder'
 
 VALUE is a symbol representing the registered CCL program used for
-decoding.  This attribute has a meaning only when `:coding-type' is
+decoding.  This attribute is meaningful only when `:coding-type' is
 `ccl'.
 
 `:ccl-encoder'
 
 VALUE is a symbol representing the registered CCL program used for
-encoding.  This attribute has a meaning only when `:coding-type' is
+encoding.  This attribute is meaningful only when `:coding-type' is
 `ccl'.
 
-:inhibit-null-byte-detection
+`:inhibit-null-byte-detection'
 
 VALUE non-nil means Emacs ignore null bytes on code detection.
 See the variable `inhibit-null-byte-detection'.  This attribute
-has a meaning only when `:coding-type' is `undecided'.
+is meaningful only when `:coding-type' is `undecided'.
 
-:inhibit-iso-escape-detection
+`:inhibit-iso-escape-detection'
 
 VALUE non-nil means Emacs ignores ISO-2022 escape sequences on
 code detection.  See the variable `inhibit-iso-escape-detection'.
-This attribute has a meaning only when `:coding-type' is
+This attribute is meaningful only when `:coding-type' is
 `undecided'.
 
-:prefer-utf-8
+`:prefer-utf-8'
 
 VALUE non-nil means Emacs prefers UTF-8 on code detection for
-non-ASCII files.  This attribute has a meaning only when
+non-ASCII files.  This attribute is meaningful only when
 `:coding-type' is `undecided'."
   (let* ((common-attrs (mapcar 'list
                                '(:mnemonic