Add option to control default outlining in 'C-h b'

* lisp/help.el (describe-bindings-outline-rules): New user option.
(describe-bindings): Use it instead of hardcoding "Key translations".
(Bug#62708)

etc/NEWS

@@ -30,6 +30,11 @@ applies, and please also update docstrings as needed.
 
 * Changes in Emacs 30.1
 
+---
+** New user option 'describe-bindings-outline-rules'.
+This user option controls outline visibility in the output buffer of
+'describe-bindings' when 'describe-bindings-outline' in non-nil.
+
 ** X selection requests are now handled much faster and asynchronously.
 This means it should be less necessary to disable the likes of
 'select-active-regions' when Emacs is running over a slow network

lisp/help.el

@@ -727,6 +727,16 @@ Return nil if KEYS is nil."
   :group 'help
   :version "29.1")
 
+(defcustom describe-bindings-outline-rules '((match-regexp . "Key translations"))
+  "Visibility rules for outlines in the output buffer of `describe-bindings'.
+This is used as the value of `outline-default-rules' in the
+output buffer of `describe-bindings' when
+`describe-bindings-outline' is non-nil, otherwise this option
+doesn't have any effect."
+  :type 'boolean
+  :group 'help
+  :version "30.1")
+
 (declare-function outline-hide-subtree "outline")
 
 (defun describe-bindings (&optional prefix buffer)
@@ -756,8 +766,7 @@ or a buffer name."
                       outline-minor-mode-use-buttons 'insert
                       ;; Hide the longest body.
                       outline-default-state 1
-                      outline-default-rules
-                      '((match-regexp . "Key translations")))
+                      outline-default-rules describe-bindings-outline-rules)
           (outline-minor-mode 1)
           (save-excursion
             (goto-char (point-min))