Require that MessageAck must prioritized over buffered messages

Add the requirement that MessageAck must be prioritized over buffered
messages. Otherwise the other communicating party may treat missing
MessageAck as a communication disruption.

Fixes #4

rst/rsmp.rst

@@ -410,6 +410,10 @@ In the event of an communication disruption the following principles applies:
 * Once communication is restored all the buffered messages are sent according to
   the communication establishment sequence.
 * The communication buffer is stored and sent using the FIFO principle.
+* The equipment must be able to prioritize and send MessageAck when needed
+  before sending buffered messages. This is needed in order to be sure
+  that the other communicating party doesn't treat the missing MessageAck
+  as a communication disruption. See section message-acknowledgement_.
 * In the event of communications failure or power outage the contens of the
   outgoing communication buffer must not be lost.
 * The internal communication buffer of the device must at a minimum be

sphinx/source/applicability/transport_of_data.rst

@@ -174,6 +174,10 @@ In the event of an communication disruption the following principles applies:
 * Once communication is restored all the buffered messages are sent according to
   the communication establishment sequence.
 * The communication buffer is stored and sent using the FIFO principle.
+* The equipment must be able to prioritize and send MessageAck when needed
+  before sending buffered messages. This is needed in order to be sure
+  that the other communicating party doesn't treat the missing MessageAck
+  as a communication disruption. See section :ref:`message-acknowledgement`.
 * In the event of communications failure or power outage the contens of the
   outgoing communication buffer must not be lost.
 * The internal communication buffer of the device must at a minimum be