Improve Custom menu labels for 2 options

* lisp/dired.el (dired-use-ls-dired):
* lisp/progmodes/xref.el (xref-prompt-for-identifier): Improve
the doc string and the defcustom menu/tags text.  (Bug#32756)

lisp/dired.el

@@ -88,9 +88,11 @@ If nil, `dired-listing-switches' is used."
 
 (defcustom dired-use-ls-dired 'unspecified
   "Non-nil means Dired should pass the \"--dired\" option to \"ls\".
-The special value of `unspecified' means to check explicitly, and
-save the result in this variable.  This is performed the first
-time `dired-insert-directory' is called.
+If nil, don't pass \"--dired\" to \"ls\".
+The special value of `unspecified' means to check whether \"ls\"
+supports the \"--dired\" option, and save the result in this
+variable.  This is performed the first time `dired-insert-directory'
+is invoked.
 
 Note that if you set this option to nil, either through choice or
 because your \"ls\" program does not support \"--dired\", Dired
@@ -104,9 +106,10 @@ This is used by default on MS Windows, which does not have an \"ls\" program.
 Note that `ls-lisp' does not support as many options as GNU ls, though.
 For more details, see Info node `(emacs)ls in Lisp'."
   :group 'dired
-  :type '(choice (const :tag "Check for --dired support" unspecified)
+  :type '(choice (const :tag
+                        "Use --dired only if 'ls' supports it" unspecified)
                  (const :tag "Do not use --dired" nil)
-                 (other :tag "Use --dired" t)))
+                 (other :tag "Always use --dired" t)))
 
 (defcustom dired-chmod-program "chmod"
   "Name of chmod command (usually `chmod')."

lisp/progmodes/xref.el

@@ -323,21 +323,23 @@ backward."
 (defcustom xref-prompt-for-identifier '(not xref-find-definitions
                                             xref-find-definitions-other-window
                                             xref-find-definitions-other-frame)
-  "When t, always prompt for the identifier name.
+  "If non-nil, prompt for the identifier to find.
+
+When t, always prompt for the identifier name.
 
 When nil, prompt only when there's no value at point we can use,
 or when the command has been called with the prefix argument.
 
-Otherwise, it's a list of xref commands which will prompt
-anyway (the value at point, if any, will be used as the default).
-
+Otherwise, it's a list of xref commands which will always prompt,
+with the identifier at point, if any, used as the default.
 If the list starts with `not', the meaning of the rest of the
-elements is negated."
-  :type '(choice (const :tag "always" t)
-                 (const :tag "auto" nil)
-                 (set :menu-tag "command specific" :tag "commands"
+elements is negated: these commands will NOT prompt."
+  :type '(choice (const :tag "Always prompt for identifier" t)
+                 (const :tag "Prompt if no identifier at point" nil)
+                 (set :menu-tag "Prompt according to command"
+                      :tag "Prompt according to command"
 		      :value (not)
-		      (const :tag "Except" not)
+		      (const :tag "Except for commands listed below" not)
 		      (repeat :inline t (symbol :tag "command")))))
 
 (defcustom xref-after-jump-hook '(recenter