Document the `services.groups` client configuration option

This option is being added initially to support the LMS application
where it is important that students annotate in the correct group for
the course that the active assignment belongs to.

docs/publishers/config.rst

@@ -186,6 +186,15 @@ loads.
       group. The image should be suitable for display at 16x16px and the
       recommended format is SVG.
 
+   .. option:: groups
+
+      ``String[]|null``. An array of group IDs. If provided, the list of groups
+      fetched from the API will be filtered against this list so that the user
+      can only select from these groups.
+
+      This can be useful in contexts where it is important that annotations
+      are made in a particular group.
+
    .. option:: onLoginRequest
 
      ``function``. A JavaScript function that the Hypothesis client will