AR: Clarify message register uses.

aampostincrement implies the implementation does not use simple MRs.

debug_module.tex

@@ -318,6 +318,10 @@ \section{Message Registers}
 allowing for different implementations. They exist to let two sides communicate
 when the two sides already know who is the sender and who is the receiver.
 
+Only registers which are explicitly described as MRs are MRs.  \RdmDataZero
+through \RdmDataEleven are MRs, used to let the debugger pass arguments to
+abstract commands, and let the DM pass results from abstract commands back.
+
 An MR implements read and write operations on two sides.  When one side reads an
 MR, and the last write was by the other side, then the result value of the read
 is the value last written by the other side.  When one side reads an MR, and the

xml/abstract_commands.xml

@@ -258,6 +258,11 @@
             {\tt arg1} (which contains the address used) by the number of bytes
             encoded in \FacAccessmemoryAamsize.
 
+            In order for the DM to increment {\tt arg1} and read back the
+            incremented value, the relevant {\tt data} registers must be
+            readable by the DM after modification. They can not be simple
+            Message Registers.
+
             Supporting this variant is optional, but highly recommended for
             performance reasons.
         </field>