* lisp/custom.el (defgroup): Document the :prefix keyword.

diff --git a/lisp/custom.el b/lisp/custom.el
index 858b158051cdc4d810bc19585c01cc690ec3bcd9..cc817403871a58355518e3fd16a37dc23b55ca1c 100644 (file)
--- a/lisp/custom.el
+++ b/lisp/custom.el
@@ -507,7 +507,11 @@ The remaining arguments should have the form
    [KEYWORD VALUE]...
 
 For a list of valid keywords, see the common keywords listed in
-`defcustom'.
+`defcustom'.  The keyword :prefix can only be used for
+customization groups, and means that the given string should be
+removed from variable names before creating unlispified names,
+when the user option `custom-unlispify-remove-prefixes' is
+non-nil.
 
 See Info node `(elisp) Customization' in the Emacs Lisp manual
 for more information."