bug#71779: 30.0.60; tab-bar-select-restore-windows: docstring vs default value

[Kévin Le Gouguec]

Heya!

Intrigued by tab-bar-select-restore-windows's docstring:

> Function called when selecting a tab to handle windows whose buffer was 
> killed.
> When a tab-bar tab displays a window whose buffer was killed since
> this tab was last selected, this function determines what to do with
> that window.  By default, either a random buffer is displayed instead of
                ^^^^^^^^^^
> the killed buffer, or the window gets deleted.  However, with the help
> of ‘window-restore-killed-buffer-windows’ it’s possible to handle such
> situations better by displaying an information about the killed buffer.

Over here, 'emacs -Q' suggests that _by default_, this option is set to
the eponymous symbol 'tab-bar-select-restore-windows'.  So the default
behaviour that I observe, when coming back to a tab that used to show a
killed buffer, is a special-mode buffer named " *Old buffer foo*" saying
"This window displayed the buffer ‘foo’."

Based on the NEWS entry, I guess the effective default behaviour is the
one intended?  In which case, I'm attaching a suggested rewording of the
docstring, based on understanding gleaned from cross-referencing the
NEWS entry, the docstrings for set-window-configuration,
window-restore-killed-buffer-windows, the corresponding manual entries,
and the source for set-window-configuration.

I am not 100% sure I succeeded in capturing the state of things, so feel
free to dismiss the patch; my main goal was to make it easier for Past
Me to get an answer to the question: "How do I get the previous
behaviour back"?

(Rationale FWIW: IME killing buffers is deliberate, so the placeholder
buffer feels redundant, i.e. it brings no information; if I am
displeased with whatever random buffer was picked to show in that
window, I can just switch to another buffer or delete the window)

diff --git a/lisp/tab-bar.el b/lisp/tab-bar.el
index 3401b796cac..58d2ef650a1 100644
--- a/lisp/tab-bar.el
+++ b/lisp/tab-bar.el
@@ -1456,10 +1456,18 @@ tab-bar-select-restore-windows
   "Function called when selecting a tab to handle windows whose buffer was 
killed.
 When a tab-bar tab displays a window whose buffer was killed since
 this tab was last selected, this function determines what to do with
-that window.  By default, either a random buffer is displayed instead of
-the killed buffer, or the window gets deleted.  However, with the help
-of `window-restore-killed-buffer-windows' it's possible to handle such
-situations better by displaying an information about the killed buffer."
+that window.  By default, a placeholder buffer is displayed in that
+window to give the user information about the killed buffer; this
+option can also be set to:
+
+ * nil:        no special handling; `set-window-configuration' will
+               decide what to do with the window, e.g. make it display
+               another buffer;
+
+ * a function: display another buffer in that window, and pass that
+               buffer to the function.  See
+               `window-restore-killed-buffer-windows' for the calling
+               convention."
   :type '(choice (const :tag "No special handling" nil)
                  (const :tag "Show placeholder buffers"
                         tab-bar-select-restore-windows)