Documentation: hints for sending patches inline with Thunderbird

Documentation: hints for sending patches inline with Thunderbird

The standard reference for this information is the article
"Plain text e-mail - Thunderbird#Completely_plain_email" at
kb.mozillazine.org, but the hints hidden away in git's
SubmittingPatches file are more complete.  Move them to the
"git format-patch" manual so they can be installed with git and
read by a wide audience.

While at it, make some tweaks:

 - update "Approach #1" so it might work with Thunderbird 3;
 - remove ancient version numbers from the descriptions of both
   approaches so current readers might have more reason to
   complain if they don't work.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>