bug#51247: 28.0.60; Insufficient documentation of tab-bar.el internal functions

[Eli Zaretskii]

> From: Juri Linkov <juri@linkov.net>
> Cc: 51247@debbugs.gnu.org
> Date: Tue, 19 Oct 2021 09:49:59 +0300
> 
> >> >> It will display tabs or buttons from these menu items.
> >> >
> >> > When will it be tabs and when will it be buttons?
> >>
> >> Tabs are buttons that switch window configurations.
> >
> > But how do I explain in the doc string to the future programmers what
> > they should do to get tabs and what they should do to get buttons that
> > drop a menu?
> 
> You don't need to explain about buttons that drop a menu.
> This is irrelevant for the description of tab-bar-format.
> Just imagine there is no tab-bar-format-menu-global
> that uses own unrelated menu items.

Sorry, that won't fly.  The doc string should explain enough for other
Lisp programmers to be able to produce buttons that drop down menus.

So please humor me with a detailed enough answer to my questions, I'm
investing enough energy in this stuff to be entitled to a bit more
than a couple of dismissive sentences.

We must have a decent doc string for this defcustom.  Please help me
fix what we have now.