:group keyword for define-derived-mode

:group keyword for define-derived-mode

[Luc Teirlinck]

The doc for the :group keyword of define-minor-mode does not say
anything about what it is going to be used for:

group GROUP
      Declare the customization group that corresponds to this mode.

The patches below explain it concisely for the docstring and in
somewhat more detail for the Elisp manual.  I can install if desired.

It is only used for the command `customize-mode'.  I wonder what the
status of this command is.  It is not documented in either the Emacs
and Elisp manual, nor in the NEWS.  Due to several problems, it does
not appear ready to be advertised to general users.  That is why the
proposed Elisp manual text describes it as "still experimental and
unadvertised".

===File ~/derived-diff======================================
*** derived.el	12 May 2005 20:37:22 -0500	1.5
--- derived.el	19 May 2005 19:29:50 -0500	
***************
*** 132,137 ****
--- 132,138 ----
    arguments are currently understood:
  :group GROUP
  	Declare the customization group that corresponds to this mode.
+ 	The command `customize-mode' uses this.
  :syntax-table TABLE
  	Use TABLE instead of the default.
  	A nil value means to simply use the same syntax-table as the parent.
============================================================

===File ~/modes.texi-diff===================================
*** modes.texi	19 May 2005 17:41:33 -0500	1.107
--- modes.texi	19 May 2005 19:21:43 -0500	
***************
*** 1032,1040 ****
  are evaluated.  The following keywords are currently supported:
  
  @table @code
- @item :group
- If this is specified, it is the customization group for this mode.
- 
  @item :syntax-table
  You can use this to explicitly specify a syntax table for the new
  mode.  If you specify a @code{nil} value, the new mode uses the same
--- 1032,1037 ----
***************
*** 1049,1054 ****
--- 1046,1059 ----
  abbrev table as @var{parent}, or @code{fundamental-mode-abbrev-table}
  if @var{parent} is @code{nil}.  (Again, a @code{nil} value is
  @emph{not} equivalent to not specifying this keyword.)
+ 
+ @item :group
+ If this is specified, the value should be the customization group for
+ this mode.  (Not all major modes have one.)  The value is recorded in
+ the @code{custom-mode-group} property of the major mode command
+ symbol.  Only the (still experimental and unadvertised) command
+ @code{customize-mode} currently uses this.  @code{define-derived-mode}
+ does @emph{not} automatically define the specified customization group.
  @end table
  
  Here is a hypothetical example:
============================================================