[llvm][docs] commit phabricator patch

Users upgrading to PHP 8.1 might start observing failures with `arc`.
Commit @ychen's suggestions as a patch in tree that can be applied since
arcanist is no longer accepting patches.

Also, remove the suggestion to apply an external patch updating CA
certs. It seems that this was fixed in upstream arcanist before they
stopped accepting patches. Compare
https://github.com/rashkov/arcanist/commit/e3659d43d8911e91739f3b0c5935598bceb859aa
vs
https://github.com/rashkov/arcanist/commit/13d3a3c3b100979c34dda261fe21253e3571bc46

Link: https://secure.phabricator.com/book/phabcontrib/article/contributing_code/

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D129232