m.room.power_levels: clarify documentation

- clarify description and add text for invite, ban, etc.
- the keys aren't required. Document their defaults
- add 'invite' to example

changelogs/client_server.rst

@@ -7,6 +7,7 @@
     federated aliases
   - ``GET /directory/room/{roomAlias}`` cannot return a 409; the ``PUT``
     endpoint can, however.
+  - Clarify the behaviour of the ``m.room.power_levels`` event.
 
 r0.1.0
 ======