fix TM2p: remove sentence about populating version from channelSerial

this doesn't work because channelSerial is regional, version must be the origin
serial, i.e. populated from the serial in the publishing region.

For now the server will just always populate version if it should be set
(currently only in mutableMessages namespaces)

textile/features.textile

@@ -1344,8 +1344,8 @@ h4. Message
 ** @(TM2i)@ @extras@ JSON-encodable object, used to contain any arbitrary key value pairs which may also contain other primitive JSON types, JSON-encodable objects or JSON-encodable arrays. The @extras@ field is provided to contain message metadata and/or ancillary payloads in support of specific functionality, e.g. push. Each of these supported extensions is documented separately; for 1.1 the only supported extension is @push@, via the @extras.push@ member; 1.2 adds the @delta@ extension whose keys and values are described by the attributes of the type @DeltaExtras@, and the @headers@ extension, which contains arbitrary @string->string@ key-value pairs, settable at publish time, and @ref@ whose keys and values are described by the attributes of the type @ReferenceExtras@. Unless otherwise specified, the client library should not attempt to do any filtering or validation of the @extras@ field itself, but should treat it opaquely, encoding it and passing it to realtime unaltered.
 ** @(TM2f)@ @timestamp@ time in milliseconds since epoch. If a message received from Ably over a realtime transport does not contain a @timestamp@, the SDK must set it to the @timestamp@ of the encapsulating @ProtocolMessage@
 ** @(TM2j)@ @action@ enum
-** @(TM2p)@ @version@ string - an opaque string that uniquely identifies the message, and is different for different versions. If a message received from Ably over a realtime transport does not contain a @version@, the SDK must set it to @<channelSerial>:<padded_index>@ from the @channelSerial@ field of the enclosing @ProtocolMessage@, and @padded_index@ is the index of the message inside the @messages@ array of the @ProtocolMessage@, left-padded with 0s to three digits (for example, the second entry might be @foo:001@).
-** @(TM2k)@ @serial@ string - an opaque string that uniquely identifies the message. If a message received from Ably (whether over realtime or REST, eg history) with an @action@ of @MESSAGE_CREATE@ does not contain a @serial@, the SDK must set it equal to its @TM2p@ @version@.
+** @(TM2p)@ @version@ string - an opaque string that uniquely identifies the message, and is different for different versions. (May not be populated).
+** @(TM2k)@ @serial@ string - an opaque string that uniquely identifies the message. If a message received from Ably (whether over realtime or REST, eg history) with an @action@ of @MESSAGE_CREATE@ does not contain a @serial@ but does contain a @version@, the SDK must set it equal to the @TM2p@ @version@. (May not be populated).
 ** @(TM2o)@ @createdAt@ time in milliseconds since epoch. If a message received from Ably (whether over realtime or REST, eg history) with an @action@ of @MESSAGE_CREATE@ does not contain a @createdAt@, the SDK must set it equal to the @TM2f@ @timestamp@.
 ** @(TM2l)@ @refSerial@ string - an opaque string that uniquely identifies some referenced message.
 ** @(TM2m)@ @refType@ string - an opaque string that identifies the type of this reference.
@@ -2212,7 +2212,7 @@ class Message: // TM*
   name: String? // TM2g
   timestamp: Time // TM2f
   action: MessageAction // TM2j
-  serial: string // TM2k
+  serial: string? // TM2k
   version: string? // TM2p
   refSerial: string? // TM2l
   refType: string? // TM2m