Documentation/githooks: Explain pre-rebase parameters

Descriptions borrowed from templates/hooks--pre-rebase.sample.

Signed-off-by: W. Trevor King <wking@tremily.us>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/githooks.txt

@@ -140,9 +140,11 @@ the outcome of 'git commit'.
 pre-rebase
 ~~~~~~~~~~
 
-This hook is called by 'git rebase' and can be used to prevent a branch
-from getting rebased.
-
+This hook is called by 'git rebase' and can be used to prevent a
+branch from getting rebased.  The hook may be called with one or
+two parameters.  The first parameter is the upstream from which
+the series was forked.  The second parameter is the branch being
+rebased, and is not set when rebasing the current branch.
 
 post-checkout
 ~~~~~~~~~~~~~