git-branch.txt: mention --set-upstream as a way to change upstream configuration

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-branch.txt

@@ -41,7 +41,8 @@ When a local branch is started off a remote-tracking branch, git sets up the
 branch so that 'git pull' will appropriately merge from
 the remote-tracking branch. This behavior may be changed via the global
 `branch.autosetupmerge` configuration flag. That setting can be
-overridden by using the `--track` and `--no-track` options.
+overridden by using the `--track` and `--no-track` options, and
+changed later using `git branch --set-upstream`.
 
 With a '-m' or '-M' option, <oldbranch> will be renamed to <newbranch>.
 If <oldbranch> had a corresponding reflog, it is renamed to match