Remove duplicate description of who can redact a message and correct that description

Signed-off-by: Aaron Raimist <aaron@raim.ist>

api/client-server/redaction.yaml

@@ -36,9 +36,9 @@ paths:
 
         This cannot be undone.
 
-        Users may redact their own events, and any user with a power level
+        Any user with a power level greater than or equal to the ``redact`` power level
-        greater than or equal to the ``redact`` power level of the room may
+        of the room may redact events there. Additionally, server administrators may
-        redact events there.
+        redact messages sent by their server.
       operationId: redactEvent
       security:
         - accessToken: []

event-schemas/schema/m.room.redaction

@@ -1,7 +1,7 @@
 ---
 allOf:
   - $ref: core-event-schema/room_event.yaml
-description: 'Redacting an event means that all keys not required by the protocol are stripped off, allowing messages to be hidden or allowing admins to remove offensive or illegal content. Users may redact their own events, and any user with a power level greater than or equal to the redact power level of the room may redact events in that room. Redactions cannot be undone. The event that has been redacted is specified in the ``redacts`` event level key.'
+description: 'This event is created by the server to describe which event has been redacted, by whom, and optionally why. The event that has been redacted is specified in the ``redacts`` event level key. Redacting an event means that all keys not required by the protocol are stripped off, allowing messages to be hidden or allowing admins to remove offensive or illegal content.'
 properties:
   content:
     properties: