doc: comment fake_event usage in meeting datetimes API

chrono/api/views.py

@@ -229,6 +229,8 @@ class MeetingDatetimes(GenericAPIView):
             entries[key] = slot
         slots = sorted(entries.values(), key=lambda x: x.start_datetime)
 
+        # create fillslot API URL as a template, to avoid expensive calls
+        # to request.build_absolute_uri()
         fake_event_pk = '__event_id__'
         fillslot_url = request.build_absolute_uri(
                 reverse('api-fillslot',