Add some words to explain why we do things the way we do

src/pushprocessor.js

@@ -25,7 +25,12 @@ const RULEKINDS_IN_ORDER = ['override', 'content', 'room', 'sender', 'underride'
 
 // The default override rules to apply when calculating actions for an event. These
 // defaults apply under no other circumstances to avoid confusing the client with server
-// state.
+// state. We do this for two reasons:
+//   1. Synapse is unlikely to send us the push rule in an incremental sync - see
+//      https://github.com/matrix-org/synapse/pull/4867#issuecomment-481446072 for
+//      more details.
+//   2. We often want to start using push rules ahead of the server supporting them,
+//      and so we can put them here.
 const DEFAULT_OVERRIDE_RULES = [
     {
         // For homeservers which don't support MSC1930 yet