Use regexp-opt-charset to improve regexp tweaks

* lisp/emacs-lisp/regexp-opt.el (regexp-opt):
Reword confusing sentence in doc string.
* lisp/erc/erc.el (erc-lurker-maybe-trim):
* lisp/mail/footnote.el (footnote-hebrew-numeric-regex):
Improve by using regexp-opt-charset.
1 job for master in 60 minutes and 1 second (queued for 3 seconds)
Status Job ID Name Coverage
failed #1151


Name Stage Failure
test-all Test The script exceeded the maximum execution time set for the job
make -C test check-expensive
make[1]: Entering directory '/builds/emacs/emacs/test'
rm -f ./*.tmp
make[2]: Entering directory '/builds/emacs/emacs/test'
make[3]: Entering directory '/builds/emacs/emacs/test'
ELC lisp/net/tramp-tests.elc
GEN lisp/net/tramp-tests.log
Pulling docker image gitlab/gitlab-runner-helper:x86_64-f100a208 ...
ERROR: Job failed: execution took longer than 1h0m0s seconds