add comments explaining prefetch need

lms/djangoapps/courseware/views.py

@@ -327,6 +327,9 @@ def progress(request, course_id, student_id=None):
 
     # NOTE: To make sure impersonation by instructor works, use
     # student instead of request.user in the rest of the function.
+
+    # The pre-fetching of groups is done to make auth checks not require an 
+    # additional DB lookup (this kills the Progress page in particular).
     student = User.objects.prefetch_related("groups").get(id=student.id)
 
     student_module_cache = StudentModuleCache.cache_for_descriptor_descendents(