Make C-h S in Customize buffers work more reliably

* lisp/cus-edit.el (custom-unlispify-menu-entry): Put a property
on the returned string so that we can determine what the symbol
was after the fact.

* lisp/info-look.el (info-lookup-guess-default*): Allow lambda
forms as rules.
(:mode): Use the stored symbol.
(info-lookup-guess-custom-symbol): No longer used; mark as
obsolete (bug#41905).

lisp/cus-edit.el

@@ -561,7 +561,7 @@ value unless you are sure you know what it does."
 	   (unless no-suffix
 	     (goto-char (point-max))
 	     (insert "..."))
-	   (buffer-string)))))
+	   (propertize (buffer-string) 'custom-data symbol)))))
 
 (defcustom custom-unlispify-tag-names t
   "Display tag names as words instead of symbols if non-nil."

lisp/info-look.el

@@ -557,7 +557,7 @@ Return nil if there is nothing appropriate in the buffer near point."
 		  (info-lookup->regexp topic mode)))
 	(start (point)) end regexp subexp result)
     (save-excursion
-      (if (symbolp rule)
+      (if (functionp rule)
 	  (setq result (funcall rule))
 	(if (consp rule)
 	    (setq regexp (car rule)
@@ -610,6 +610,7 @@ Return nil if there is nothing appropriate in the buffer near point."
 
 (defun info-lookup-guess-custom-symbol ()
   "Get symbol at point in custom buffers."
+  (declare (obsolete nil "28.1"))
   (condition-case nil
       (save-excursion
 	(let ((case-fold-search t)
@@ -1065,7 +1066,9 @@ Return nil if there is nothing appropriate in the buffer near point."
  :mode 'Custom-mode
  :ignore-case t
  :regexp "[^][()`'‘’,:\" \t\n]+"
- :parse-rule 'info-lookup-guess-custom-symbol
+ :parse-rule (lambda ()
+               (when-let ((symbol (get-text-property (point) 'custom-data)))
+                 (symbol-name symbol)))
  :other-modes '(emacs-lisp-mode))
 
 (info-lookup-maybe-add-help