Document nil value of message-signature and mail-signature

* lisp/gnus/message.el (message-signature): Allow nil as a
customizable value, and describe its effect.
* lisp/mail/sendmail.el (mail-signature): Describe effect of nil
value (bug#32680).
1 job for master in 90 minutes and 1 second (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #1745
test-all

01:30:01

 
Name Stage Failure
failed
test-all Test The script exceeded the maximum execution time set for the job
  GEN      lisp/international/ccl-tests.log
ELC lisp/international/mule-tests.elc
GEN lisp/international/mule-tests.log
ELC lisp/international/mule-util-tests.elc
GEN lisp/international/mule-util-tests.log
ELC lisp/international/ucs-normalize-tests.elc
GEN lisp/international/ucs-normalize-tests.log
Pulling docker image gitlab/gitlab-runner-helper:x86_64-f100a208 ...
ERROR: Job failed: execution took longer than 1h30m0s seconds