describe-keymap: Suggest symbol at point

* lisp/help-fns.el (describe-keymap): Suggest symbol at point
if it is a keymap.  (Bug#55393)
* etc/NEWS: Announce change in behavior of 'describe-keymap'.

etc/NEWS

@@ -562,6 +562,11 @@ minor modes are listed after the major mode.
 The apropos commands will now select the apropos window if
 'help-window-select' is non-nil.
 
+---
+*** 'describe-keymap' now considers the symbol at point.
+If the symbol at point is a keymap, 'describe-keymap' suggests it as
+the default candidate.
+
 ** Outline Mode
 
 +++

lisp/help-fns.el

@@ -1922,7 +1922,10 @@ in `describe-keymap'.  See also `Searching the Active Keymaps'."
 When called interactively, prompt for a variable that has a
 keymap value."
   (interactive
-   (let* ((km (help-fns--most-relevant-active-keymap))
+   (let* ((sym (symbol-at-point))
+          (km (or (and (keymapp (ignore-errors (symbol-value sym)))
+                       sym)
+                  (help-fns--most-relevant-active-keymap)))
           (val (completing-read
                 (format-prompt "Keymap" km)
                 obarray