Document new "already-merged" rule for branch -d

Document new "already-merged" rule for branch -d

v1.7.0-rc0~18^2 (branch -d: base the "already-merged" safety on the
branch it merges with, 2009-12-29) taught ‘git branch’ a new heuristic
for when it is safe to delete a branch without forcing the issue.  It
is safe to delete a branch "topic" without second thought if:

 - the branch "topic" is set up to pull from a (remote-tracking,
   usually) branch and is fully merged in that "upstream" branch, or

 - there is no branch.topic.merge configuration and branch "topic" is
   fully merged in the current HEAD.

Update the man page to acknowledge the new rules.

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