Always defvar a mode's hook and provide a docstring.

* lisp/emacs-lisp/easy-mmode.el (define-minor-mode):
* lisp/emacs-lisp/derived.el (define-derived-mode): Always defvar the
mode hook and provide a docstring.

lisp/ChangeLog

@@ -1,3 +1,9 @@
+2013-05-27  Stefan Monnier  <monnier@iro.umontreal.ca>
+
+	* emacs-lisp/easy-mmode.el (define-minor-mode):
+	* emacs-lisp/derived.el (define-derived-mode): Always defvar the
+	mode hook and provide a docstring.
+
 2013-05-27  Alan Mackenzie  <acm@muc.de>
 
 	Remove spurious syntax-table text properties inserted by C-y.

lisp/emacs-lisp/derived.el

@@ -192,12 +192,11 @@ See Info node `(elisp)Derived Modes' for more details."
 		     parent child docstring syntax abbrev))
 
     `(progn
-       (unless (get ',hook 'variable-documentation)
-	 (put ',hook 'variable-documentation
-	      (purecopy ,(format "Hook run when entering %s mode.
+       (defvar ,hook nil
+         ,(format "Hook run after entering %s mode.
 No problems result if this variable is not bound.
 `add-hook' automatically binds it.  (This is true for all hook variables.)"
-		       name))))
+                  name))
        (unless (boundp ',map)
 	 (put ',map 'definition-name ',child))
        (with-no-warnings (defvar ,map (make-sparse-keymap)))

lisp/emacs-lisp/easy-mmode.el

@@ -296,6 +296,12 @@ the mode if ARG is omitted or nil, and toggle it if ARG is `toggle'.
        ;; up-to-here.
        :autoload-end
 
+       (defvar ,hook nil
+         ,(format "Hook run after entering or leaving `%s'.
+No problems result if this variable is not bound.
+`add-hook' automatically binds it.  (This is true for all hook variables.)"
+                  mode))
+
        ;; Define the minor-mode keymap.
        ,(unless (symbolp keymap)	;nil is also a symbol.
 	  `(defvar ,keymap-sym