Backport: Don't autoload erc-modules

Backport: Don't autoload erc-modules

* doc/misc/erc.texi (Modules): Recommend using `describe-variable'
instead of `customize-option' because the latter needs the symbol to be
loaded.
* lisp/erc/erc.el (erc-modules): Remove autoload cookie because it
caused customizations for this option to load the main library.  This
reverts the thrust of bb894845 "Teach customize-option about
erc-modules", which was added in ERC 5.6 and Emacs 30.  The motivation
for the original offending change was to allow new users to run M-x
customize-option RET erc-modules RET immediately after startup instead
of M-x customize-group RET, followed by an I-search.  (Bug#73812)

(Cherry picked from commit 1854f2751e3f73e1e5f12f6de993b6357de1766b)

Do not merge to master.