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

[Juri Linkov]

>> You assumed in the doc string that those menu items as used
>> to display a menu, whereas in fact those menu items as used
>> to display tabs and other buttons on the tab bar.
>
> I'm trying to understand the difference between the two kinds of
> menus, the one which causes the tab bar to display tabs, vs the one
> that causes the display of a single tab that drops down a menu.
> Please help me understand that difference.

The tab bar is created from the list where elements are either a string,
or a menu-item with a title and a binding.  The title is displayed as
a button on the tab bar.  Clicking on such button calls the binding.
The binding usually is a command that switches tabs.  But the binding
can do everything.

To make the distinction more clear, I created a new command
`tab-bar-menu-bar' that is bound to the button created by
`tab-bar-format-menu-bar'.