apply changes from review

event-schemas/schema/m.room_key.withheld

@@ -57,8 +57,17 @@ properties:
           sender.
       code:
         type: string
+        enum:
+          - m.blacklisted
+          - m.unverified
+          - m.unauthorised
+          - m.unavailable
+          - m.no_olm
         description: |-
-          A machine-readable code for why the key was not sent.
+          A machine-readable code for why the key was not sent. Codes beginning
+          with `m.` are reserved for codes defined in the Matrix
+          specification. Custom codes must use the Java package naming
+          convention.
       reason:
         type: string
         description: |-

specification/modules/end_to_end_encryption.rst

@@ -1341,10 +1341,11 @@ Example response:
 Reporting that decryption keys are withheld
 -------------------------------------------
 
-When sending an encrypted event to a room, a client can signal to other devices
+When sending an encrypted event to a room, a client can optionally signal to
-in that room that it is not sending them the keys needed to decrypt the event.
+other devices in that room that it is not sending them the keys needed to
-In this way, the receiving client can indicate to the user why it cannot
+decrypt the event.  In this way, the receiving client can indicate to the user
-decrypt the event, rather than just showing a generic error message.
+why it cannot decrypt the event, rather than just showing a generic error
+message.
 
 In the same way, when one device requests keys from another using `Key
 requests`_, the device from which the key is being requested may want to tell