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

Documentation/git-branch.txt

@@ -63,7 +63,9 @@ way to clean up all obsolete remote-tracking branches.
 OPTIONS
 -------
 -d::
-	Delete a branch. The branch must be fully merged in HEAD.
+	Delete a branch. The branch must be fully merged in its
+	upstream branch, or in `HEAD` if no upstream was set with
+	`--track` or `--set-upstream`.
 
 -D::
 	Delete a branch irrespective of its merged status.