bug#31311: 27.0; doc of `pcase'

[Noam Postavsky]

Thien-Thi Nguyen <ttn@gnu.org> writes:

> I'd like to merge that branch into ‘emacs-26’ immediately.  Any
> objections?

Nothing more from me, except a minor note about the commit message
formatting:

    * doc/lispref/elisp.texi (Top) (detailmenu)
    [Control Structures]: Add "Pattern-Matching Conditional".
    [Conditionals]: Delete section.

Square brackets are for conditional changes [1], I think you want round
parens or possibly angle brackets [2].

[1]: https://www.gnu.org/prep/standards/html_node/Conditional-Changes.html
[2]: 
https://www.gnu.org/prep/standards/html_node/Indicating-the-Part-Changed.html