git-svn: Document branches with at-sign(@).

git-svn: Document branches with at-sign(@).

git svn sometimes creates branches with an at-sign in the name
(branchname@revision). These branches confuse many users and it is a FAQ
why they are created. Document when git svn creates them.

Signed-off-by: Sebastian Leske <sebastian.leske@sleske.name>
Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>