The hints in SubmittingPatches about stopping GMail from clobbering
patches are widely useful both as examples of "git send-email" and
"git imap-send" usage.

Move the documentation to the appropriate places.

While at it, don't encourage storing passwords in config files.
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>