doc: Add instructions for configuring Emacs smtpmail

OAuth token broker usage with cloud_mdir_sync works well with Emacs
smtpmail client. Store the access token using libsecret with Emacs whenever
a mail needs to be sent, so smtpmail can access the token using
auth-sources.

Signed-off-by: Rahul Rameshbabu <sergeantsagara@protonmail.com>