bug#19461: 25.0.50; doc of `display-buffer-*' functions that take ALIST arg

[Eli Zaretskii]

> From: martin rudalics <rudalics@gmx.at>
> Date: Sat, 3 Aug 2019 09:58:18 +0200
> Cc: 19461@debbugs.gnu.org
> 
> This problem is with us since day one of that buffer display code.
> It's probably due to the fact that the 'display-buffer-*' functions
> are inherently internal but never have been marked as such in order to
> describe them in the manual and allow users to add new ones.

What are the non-internal functions which should be called instead of
these ones (and will call these ones when needed)?

>  > At least one of these doc strings
>  > should say what it is, and the rest could refer to it.
> 
> Saying what it is is non-trivial, see section 28.13.3 of the Elisp
> manual.  So I have no good idea how to do what you propose.

I gave it a go on the master branch, please take a look.