Teach customize-option about erc-modules

Teach customize-option about erc-modules

* lisp/erc/erc-goodies.el (erc-scrolltobottom-mode)
(erc-scrolltobottom-enable): Use `setq' instead of `setopt' because
the latter isn't defined in Emacs 27 and 28.  This fix is unrelated to
the main thrust of this commit.
* lisp/erc/erc.el (erc-modules): Make good on decades old language in
info node "(erc) Modules" by ensuring `customize-option' can find this
option before its containing library is loaded.  Like
`gnus-select-method', this option serves as an entry point for
configuring the application and is presented that way in tutorials and
library front matter.  Moreover, it can't be reasonably autoloaded in
the traditional way because of its many dependencies and large textual
footprint.
(erc-display-message): Revise doc string.