General improvements to the push rules module

This commit does a few things:
* Add 3 undocumented push rules to the spec for encrypted events and at-room notifications.
* Require unrecognized conditions to not match, ensuring that future conditions do not cause clients to accidentally notify users.
* Clarify that push rules should be enabled when created.
* Document a new condition required for at-room notifications.

Fixes https://github.com/matrix-org/matrix-doc/issues/1163
Fixes https://github.com/matrix-org/matrix-doc/issues/1034
Fixes https://github.com/matrix-org/matrix-doc/issues/676
Fixes https://github.com/matrix-org/matrix-doc/issues/1033
Relates to https://github.com/matrix-org/matrix-doc/issues/1101

api/client-server/pushrules.yaml

@@ -343,6 +343,8 @@ paths:
         This endpoint allows the creation, modification and deletion of pushers
         for this user ID. The behaviour of this endpoint varies depending on the
         values in the JSON body.
+
+        When creating push rules, they MUST be enabled by default.
       operationId: setPushRule
       security:
         - accessToken: []
@@ -424,7 +426,7 @@ paths:
             required: ["actions"]
       responses:
         200:
-          description: The pusher was set.
+          description: The push rule was created/updated.
           examples:
             application/json: {
               }