Mention if an ERC module is local in its doc string

Mention if an ERC module is local in its doc string

* lisp/erc/erc-common.el (erc--assemble-toggle)
(define-erc-module): Update language of doc string to indicate if a
module is local.
* test/lisp/erc/erc-tests.el (define-erc-module--global)
(define-erc-module--local)
(define-erc-module--local/permanent-locals): Update expected output.

(cherry picked from commit ab78cbfabc856b0f9c069ab42ad8827cdaa499ce)