Merge branch 'ag/doc-send-email'

Documentation updates for "git send-email".

* ag/doc-send-email:
  docs: mention possible options for Proton Mail users
  docs: add a paragraph explaining the `sendmailCmd` option of sendemail
  docs: add an OAuth2.0 credential helper for AOL accounts
  docs: add outlookidfix config option to sendemail documentation
  docs: link OpenSSL's verify(1) manual page to know about -CAfile and -CApath options