Restore IDs of deleted spec points

These spec points were deleted in 6118c46; restore their IDs so that we don’t
accidentally reuse these identifiers, and so that these don’t look like invalid
IDs in the Swift repo (where these spec points had been implemented).

I’ve used the same language as in CONTRIBUTING.md for deleting these spec
points (so that automated tools know they should consider these spec points as
deleted), but I don’t have anything to put for the “it was valid up to and
including” part.

textile/chat-features.textile

@@ -196,6 +196,8 @@ Broadly speaking, messages are published via REST calls to the Chat HTTP API and
 * @(CHA-M3)@ Messages are sent to Ably via the Chat REST API, using the @send@ method.
 ** @(CHA-M3a)@ @[Testable]@ When a message is sent successfully, the caller shall receive a struct representing the "@Message@":#chat-structs-message in response (as if it were received via Realtime event).
 ** @(CHA-M3b)@ @[Testable]@ A message may be sent without @metadata@ or @headers@. When these are not specified by the user, they must be omitted from the REST payload.
+** @(CHA-M3c)@ This clause has been deleted.
+** @(CHA-M3d)@ This clause has been deleted.
 ** @(CHA-M3e)@ @[Testable]@ If an error is returned from the REST API, its @ErrorInfo@ representation shall be thrown as the result of the @send@ call.
 * @(CHA-M4)@ Messages can be received via a subscription in realtime.
 ** @(CHA-M4a)@ @[Testable]@ A subscription can be registered to receive incoming messages. Adding a subscription has no side effects on the status of the room or the underlying realtime channel.
@@ -233,6 +235,8 @@ All ephemeral room reactions are handled over the Realtime connection.
 * @(CHA-ER2)@ A @Reaction@ corresponds to a single reaction in a chat room. This is analogous to a single user-specified message on an Ably channel (NOTE: **not** a @ProtocolMessage@).
 * @(CHA-ER3)@ Ephemeral room reactions are sent to Ably via the Realtime connection via a @send@ method.
 ** @(CHA-ER3a)@ @[Testable]@ Reactions are sent on the channel using a message in "this format":#realtime-room-reactions.
+** @(CHA-ER3b)@ This clause has been deleted.
+** @(CHA-ER3c)@ This clause has been deleted.
 * @(CHA-ER4)@ A user may subscribe to reaction events in Realtime.
 ** @(CHA-ER4a)@ @[Testable]@ A user may provide a listener to subscribe to reaction events. This operation must have no side-effects in relation to room or underlying status. When a "realtime message":#realtime-room-reactions with name @roomReaction@ is received, this message is converted into a "reaction object":#chat-structs-ephemeral-reactions and emitted to subscribers.
 ** @(CHA-ER4b)@ @[Testable]@ A user may unsubscribe a registered listener. This operation must have no side-effects in relation to room or underlying status. Once unsubscribed, subsequent reaction events must not be emitted to this listener.