Add a new permission to permit rebasing changes in the web UI

So far only the change owner and submitters are permitted to rebase a
change in the web UI by pushing the 'Rebase Change' button. This change
introduces a new access right that can be assigned to other users to
permit them to do the rebase in the web UI as well.

Change-Id: I21e9981bd685f1d69e792abbd7eeaf1a60d039b8
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

Documentation/access-control.txt

diff --git a/Documentation/access-control.txt b/Documentation/access-control.txt
index fa727b5..5172e03 100644
--- a/Documentation/access-control.txt
+++ b/Documentation/access-control.txt
@@ -703,6 +703,21 @@
 is already restricted to the correct set of users.
 
 
+[[category_rebase]]
+Rebase
+~~~~~~
+
+This category permits users to rebase changes via the web UI by pushing
+the `Rebase Change` button.
+
+The change owner and submitters can always rebase changes in the web UI
+(even without having the `Rebase` access right assigned).
+
+Users without this access right who are able to upload new patch sets
+can still do the rebase locally and upload the rebased commit as a new
+patch set.
+
+
 [[category_submit]]
 Submit
 ~~~~~~