Document project-kill-buffers-display-buffer-list

* doc/emacs/maintaining.texi (Project Buffer Commands): Mention
project-kill-buffers-display-buffer-list.

* lisp/progmodes/project.el (project-kill-buffers): Link to
project-kill-buffers-display-buffer-list.

doc/emacs/maintaining.texi

@@ -1834,11 +1834,14 @@ buffers as candidates for completion.
 
 @findex project-kill-buffers
 @vindex project-kill-buffer-conditions
+@vindex project-kill-buffers-display-buffer-list
   When you finish working on the project, you may wish to kill all the
 buffers that belong to the project, to keep your Emacs session
 smaller.  The command @kbd{C-x p k} (@code{project-kill-buffers})
 accomplishes that: it kills all the buffers that belong to the current
-project that satisfy any of @code{project-kill-buffer-conditions}.
+project that satisfy any of @code{project-kill-buffer-conditions}.  If
+@code{project-kill-buffers-display-buffer-list} is non-@code{nil}, the
+buffers to be killed will be displayed first.
 
 @node Switching Projects
 @subsection Switching Projects

etc/NEWS

@@ -927,6 +927,7 @@ it with new 'term-{faint,italic,slow-blink,fast-blink}' faces.
 *** 'project-find-file' and 'project-or-external-find-file' now accept
 a prefix argument which is interpreted to mean "include all files".
 
++++
 *** 'project-kill-buffers' can display the list of buffers to kill.
 Customize the user option 'project-kill-buffers-display-buffer-list'
 to enable the display of the buffer list.

lisp/progmodes/project.el

@@ -1330,7 +1330,9 @@ identical.  Only the buffers that match a condition in
 `project-kill-buffer-conditions' will be killed.  If NO-CONFIRM
 is non-nil, the command will not ask the user for confirmation.
 NO-CONFIRM is always nil when the command is invoked
-interactively."
+interactively.
+
+Also see the `project-kill-buffers-display-buffer-list' variable."
   (interactive)
   (let* ((pr (project-current t))
          (bufs (project--buffers-to-kill pr))