Update send-pack pipeline documentation.
Update send-pack pipeline documentation.

The pipeline was much more complex and needed documentation, but
now it is trivial and straightforward.

Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git