Update spec to say what the default state / events defaults are, from inspecting the behaviour of synapse (which I'm not sure was intentional).

event-schemas/schema/m.room.power_levels

@@ -1,7 +1,7 @@
 {
     "type": "object",
     "title": "Defines the power levels (privileges) of users in the room.",
-    "description": "This event specifies the minimum level a user must have in order to perform a certain action. It also specifies the levels of each user in the room. If a ``user_id`` is in the ``users`` list, then that ``user_id`` has the associated power level. Otherwise they have the default level ``users_default``. If ``users_default`` is not supplied, it is assumed to be 0. The level required to send a certain event is governed by ``events``, ``state_default`` and ``events_default``. If an event type is specified in ``events``, then the user must have at least the level specified in order to send that event. If the event type is not supplied, it defaults to ``events_default`` for Message Events and ``state_default`` for State Events.",
+    "description": "This event specifies the minimum level a user must have in order to perform a certain action. It also specifies the levels of each user in the room. If a ``user_id`` is in the ``users`` list, then that ``user_id`` has the associated power level. Otherwise they have the default level ``users_default``. If ``users_default`` is not supplied, it is assumed to be 0. The level required to send a certain event is governed by ``events``, ``state_default`` and ``events_default``. If an event type is specified in ``events``, then the user must have at least the level specified in order to send that event. If the event type is not supplied, it defaults to ``events_default`` for Message Events and ``state_default`` for State Events. If there is no `state_default` in the `power_levels` event, the `state_default` is 50. If the room contains no `power_levels` event, the `state_default` is 0. The `events_default` is 0 in either of these cases.",
     "allOf": [{
         "$ref": "core-event-schema/state_event.yaml"
     }],