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

[Juri Linkov]

>>   (add-hook 'tab-bar-format 'ignore 1)
>>
>> and everything still works fine.
>
> I'm afraid this doesn't help me.  The doc strings says:
>
>   Every item in the list is a function that returns
>   a string, or a list of menu-item elements, or nil.
>
> I now have a good idea what to say about return values that are
> strings and those that are lists of menu items.  What do I say about
> return values of nil? what is their effect on the tab bar's
> appearance?  Are you saying that there's no effect whatsoever?

The nil value is supported only for the case when a function
is on the tab-bar-format list, but its return value depends
on some variable, and on some conditions might return nil.

An example is tab-bar-format-history: when tab-bar-history-mode
is nil, then tab-bar-format-history returns nil, but it doesn't break
the tab bar.