Update docs for `customize-mode'

* lisp/cus-edit.el (customize-mode): This function works with both major
and minor modes, make docstring and prompt reflect that (Bug #23649).

* doc/lispref/modes.texi (Derived Modes): Remove note about
customize-mode being experimental; it has been around since Emacs
22 (Bug #11299).
Co-authored-by: Drew Adams <drew.adams@oracle.com>

doc/lispref/modes.texi

@@ -794,10 +794,9 @@ if @var{parent} is @code{nil}.  (Again, a @code{nil} value is
 
 @item :group
 If this is specified, the value should be the customization group for
-this mode.  (Not all major modes have one.)  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.
+this mode.  (Not all major modes have one.)  The command
+@code{customize-mode} uses this.  @code{define-derived-mode} does
+@emph{not} automatically define the specified customization group.
 @end table
 
 Here is a hypothetical example:

lisp/cus-edit.el

@@ -1072,9 +1072,10 @@ are shown; the contents of those subgroups are initially hidden."
 
 ;;;###autoload
 (defun customize-mode (mode)
-  "Customize options related to the current major mode.
-If a prefix \\[universal-argument] was given (or if the current major mode has no known group),
-then prompt for the MODE to customize."
+  "Customize options related to a major or minor mode.
+By default the current major mode is used.  With a prefix
+argument or if the current major mode has no known group, prompt
+for the MODE to customize."
   (interactive
    (list
     (let ((completion-regexp-list '("-mode\\'"))
@@ -1083,8 +1084,8 @@ then prompt for the MODE to customize."
 	  major-mode
 	(intern
 	 (completing-read (if group
-			      (format "Major mode (default %s): " major-mode)
-			    "Major mode: ")
+			      (format "Mode (default %s): " major-mode)
+			    "Mode: ")
 			  obarray
 			  'custom-group-of-mode
 			  t nil nil (if group (symbol-name major-mode))))))))