Merge branch 'ag/send-email-docs'

Documentation for "git send-email" has been updated with a bit more
credential helper and OAuth information.

* ag/send-email-docs:
  docs: make the purpose of using app password for Gmail more clear in send-email
  docs: remove credential helper links for emails from gitcredentials
  docs: improve formatting in git-send-email documentation
  docs: add credential helper for yahoo and link Google's sendgmail tool