doc: highlight streamable form of GXMT

Author: jengelh

doc/mtformat.rst

@@ -5,6 +5,13 @@ The Gromox Mailbox Transfer format (GXMT) is our streaming-capable
 serialization format for conveying a bundle of MAPI objects for the purpose of
 interprocess communication.
 
+Whereas a TNEF file only records one object, GXMT can record many, folders
+included. GXMT is streamable; there are no forward references, so the stream
+need not be buffered by a reader to resolve refs. Backreferences are typically
+named properties and folders that were created as part of processing earlier
+parts of a stream. Out of convenience of implementation, the mt2exm program
+buffers one entire object at a time before acting on it.
+
 
 Spec
 ====