Clarify timestamp behavior in protocol documentation

Updated the description of the timestamp field to indicate that it reflects the time at which the message was first received by the system, and will not be updated on subsequent message updates.

textile/protocol.textile

@@ -194,7 +194,7 @@ The members are as follows.
 Client libraries do not need to populate this field if the clientId is implicit (ie a clientId was specified when the library was initialized, and is therefore connection-wide. Also, this field will be empty if no clientId has been specified either on library initialization or when publishing the message.<br><br>
 Messages sent from the service to the client will contain a clientId if one is available.
 
-- i64 timestamp := This is the timestamp indicating the time at which the message was received by the system from the publishing client. The timestamp is included in messages sent by the service to the client. The field is expected to be empty in messages sent from a client to the service.
+- i64 timestamp := This is the timestamp indicating the time at which the message was first received by the system from a publishing client. Subsequent updates to this message shall not update the timestamp. The timestamp is included in messages sent by the service to the client. The field is expected to be empty in messages sent from a client to the service.
 
 - string or binary data := The payload of the message, binary is supported when using MessagePack.