Warn about top-level erc-update-modules calls

Warn about top-level erc-update-modules calls

* doc/misc/erc.texi (Modules): Describe unfavorable practices enacted
by third-party modules, like running `erc-update-modules' on load.
* lisp/erc/erc.el (erc-modules): Clarify comment in `custom-set'
function.
(erc--warn-about-aberrant-modules): Tweak warning message.
(erc--requiring-module-mode-p): New internal variable.
(erc--find-mode): Guard against recursive `erc-update-module'
invocations.  (Bug#57955)