Merge branch 'ag/doc-send-email'

The `send-email` documentation has been updated with OAuth2.0
related examples.

* ag/doc-send-email:
  docs: add credential helper for outlook and gmail in OAuth list of helpers
  docs: improve send-email documentation
  send-mail: improve checks for valid_fqdn

diff --cc Documentation/git-send-email.adoc
Simple merge

diff --cc git-send-email.perl
Simple merge