Documentation: publicize hints for sending patches with GMail

Documentation: publicize hints for sending patches with GMail

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: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>