Add docstrings to context menu functions, and add middle-separator

* lisp/mouse.el (context-menu-functions): Add context-menu-middle-separator
to default values.
(context-menu-middle-separator): New function.

* lisp/replace.el (occur-context-menu): Use middle-separator.

* lisp/progmodes/elisp-mode.el (elisp-context-menu):
* lisp/progmodes/prog-mode.el (prog-context-menu):
Use middle-separator and reorder menu items correspondingly.

lisp/net/dictionary.el

@@ -1376,7 +1376,7 @@ any buffer where (dictionary-tooltip-mode 1) has been called."
     (dictionary-search word)))
 
 (defun context-menu-dictionary (menu click)
-  "Dictionary context menu."
+  "Populate MENU with dictionary commands at CLICK."
   (when (thing-at-mouse click 'word)
     (define-key menu [dictionary-separator] menu-bar-separator)
     (define-key menu [dictionary-search-word-at-mouse]

lisp/net/eww.el

@@ -1027,6 +1027,7 @@ the like."
     map))
 
 (defun eww-context-menu (menu click)
+  "Populate MENU with eww commands at CLICK."
   (define-key menu [eww-separator] menu-bar-separator)
   (let ((easy-menu (make-sparse-keymap "Eww")))
     (easy-menu-define nil easy-menu nil

lisp/net/goto-addr.el

@@ -125,6 +125,7 @@ will have no effect.")
   "Keymap to hold goto-addr's mouse key defs under highlighted URLs.")
 
 (defun goto-address-context-menu (menu click)
+  "Populate MENU with goto-address commands at CLICK."
   (when (mouse-posn-property (event-start click) 'goto-address)
     (define-key menu [goto-address-separator] menu-bar-separator)
     (define-key menu [goto-address-at-mouse]