Move describe-face to the new help-fns machinery

* lisp/help-fns.el (describe-face): Move to here from faces.el and
split up (bug#36670).
(help-fns--face-custom-version-info):
(help-fns--face-attributes): Factored out into own functions.
(help-fns-describe-face-functions): New variable.

* lisp/emacs-lisp/subr-x.el (when-let): Add autoload cookie.
1 job for master in 90 minutes and 3 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Test
failed #3383
test-all

01:30:03

 
Name Stage Failure
failed
test-all Test The script exceeded the maximum execution time set for the job
  GEN      lisp/eshell/eshell-tests.log
ELC lisp/faces-tests.elc
GEN lisp/faces-tests.log
ELC lisp/ffap-tests.elc
GEN lisp/ffap-tests.log
ELC lisp/filenotify-tests.elc
GEN lisp/filenotify-tests.log
Pulling docker image gitlab/gitlab-runner-helper:x86_64-d0b76032 ...
ERROR: Job failed: execution took longer than 1h30m0s seconds