Add documentation on specifying repositories for default reviewers.

docs/manual/admin/management/default-reviewers.txt

@@ -42,8 +42,8 @@ A form will appear with the following fields:
     request when the `File regex`_ matches a file in the diff.
 
     The list contains possible review groups to match. Selected entries
-    are the groups you want to add. Hold down "Control" (on the PC) or
-    "Command" (on the Mac) to select more than one.
+    are the groups you want to add. Hold down :kbd:`Control` (on the PC) or
+    :kbd:`Command` (on the Mac) to select more than one.
 
     At least one group or one user should be selected for this default
     reviewer entry to be useful.
@@ -53,12 +53,23 @@ A form will appear with the following fields:
     when the `File regex`_ matches a file in the diff.
 
     The list contains possible users to match. Selected entries are the users
-    you want to add. Hold down "Control" (on the PC) or "Command" (on the Mac)
-    to select more than one.
+    you want to add. Hold down :kbd:`Control` (on the PC) or :kbd:`Command`
+    (on the Mac) to select more than one.
 
     At least one group or one user should be selected for this default
     reviewer entry to be useful.
 
+* **Repositories** (optional)
+    A default reviewer can be limited to one or more repositories. This is
+    particularly useful when operating a Review Board server with many
+    repositories ran by separate groups.
+
+    Hold down :kbd:`Control` (on the PC) or :kbd:`Command` (on the Mac)
+    to select more than one repository.
+
+    If no repositories are selected, then this default reviewer will apply to
+    every repository.
+
 
 .. _editing-default-reviewers: