bug#47117: 28.0.50; [PATCH] Update docstring of 'tab-line-tabs-function'

[Juri Linkov]

> The user option 'tab-line-tabs-function' on tab-line.el does not mention
> the possible value of 'tab-line-tabs-buffer-groups', which can difficult
> the discoverability of this feature for users. This patch update its
> docstring to mention the missing option, and adds a docstring to the
> function of same name. Feel free to modify the proposed patch for an
> improved and clearer docstring version.

Thanks for your patches.  There is only a very small problem:

>  (defun tab-line-tabs-buffer-groups ()
> +  "Return a list of buffers grouped either
> +by `tab-line-tabs-buffer-group-function', when set,
> +or by `tab-line-tabs-buffer-groups'."

The first line of the docstring should be a complete sentence,
as described in (info "(elisp) Function Documentation")