Document that this query can cause problems

With SQLite we're hitting it's limits, but with other databases it is
huge and slow as well.

trans/models/translation.py

@@ -537,6 +537,8 @@ class Translation(models.Model, URLMixin, PercentMixin):
             created_units.add(newunit.id)
 
         # Get lists of stale units to delete
+        # FIXME: this query can get huge, so we should find better way
+        # to delete stale units, probably sort of garbage collection
         units_to_delete = self.unit_set.exclude(
             id__in=created_units
         )