Clarify the power levels integer range

Signed-off-by: Johannes Becker <j.becker@famedly.com>

data/event-schemas/schema/m.room.power_levels.yaml

@@ -5,6 +5,11 @@ 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.
 
+  The range of power levels is specified by matrix-specific Canonical JSON
+  specification (https://spec.matrix.org/v1.3/appendices/#canonical-json), which
+  allows a range of `[-(2**53)+1, (2**53)-1]` for integers. Leading zeros are
+  not allowed.
+
   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