Documentation: Improve documentation for git-imap-send(1)

Documentation: Improve documentation for git-imap-send(1)

Change the description to be similar to that used for git-send-email(1) to
give a better description of what the tool can be used for and sound more
user-friendly.

Document the configuration variables used by git-imap-send, split the
example into tunnel and direct examples. Rephrase other parts of the
git-imap-send documentation to use better grammar and to be clearer.

Signed-off-by: Robert Shearman <robertshearman@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>