[otBase] Add comment based on review feedback

Lib/fontTools/ttLib/tables/otBase.py

@@ -1146,6 +1146,9 @@ class BaseTable(object):
         except KeyError:
             raise  # XXX on KeyError, raise nice error
         value = conv.xmlRead(attrs, content, font)
+        # Some manually-written tables have a conv.repeat of ""
+        # to represent lists. Hence comparing to None here to
+        # allow those lists to be read correctly from XML.
         if conv.repeat is not None:
             seq = getattr(self, conv.name, None)
             if seq is None: