Improve documentation of 'project-mode-line'

* etc/NEWS:
* doc/emacs/maintaining.texi (Projects):
* lisp/progmodes/project.el (project-mode-line): Improve the
documentation of 'project-mode-line'.  (Bug#66317)

doc/emacs/maintaining.texi

@@ -1743,6 +1743,14 @@ project.  Also, the VC-aware Project back-end considers ``untracked''
 files by default.  That behavior is controllable with the variable
 @code{project-vc-include-untracked}.
 
+@cindex current project name on mode line
+@defopt project-mode-line
+If this user option is non-@code{nil}, Emacs displays the name of the
+current project (if any) on the mode line; clicking @kbd{mouse-1} on
+the project name pops up the menu with the project-related commands.
+The default value is @code{nil}.
+@end defopt
+
 @menu
 * Project File Commands::   Commands for handling project files.
 * Project Buffer Commands:: Commands for handling project buffers.

etc/NEWS

@@ -890,8 +890,9 @@ showcases all their customization options.
 ** Project
 
 *** New user option 'project-mode-line'.
-When non-nil, display the name of the current project on
-the mode line.  Clicking mouse-1 pops up the project menu.
+When non-nil, display the name of the current project on the mode
+line.  Clicking 'mouse-1' on the project name pops up the project
+menu.  The default value is nil.
 
 *** New user option 'project-file-history-behavior'.
 Customizing it to 'relativize' makes commands like 'project-find-file'

lisp/progmodes/project.el

@@ -2014,9 +2014,10 @@ would otherwise have the same name."
 
 ;;;###autoload
 (defcustom project-mode-line nil
-  "Show the current project name with the menu on the mode line.
+  "Whether to show current project name and Project menu on the mode line.
 This feature requires the presence of the following item in
-`mode-line-format': `(project-mode-line project-mode-line-format)'."
+`mode-line-format': `(project-mode-line project-mode-line-format)'; it
+is part of the default mode line beginning with Emacs 30."
   :type 'boolean
   :group 'project
   :version "30.1")