docs/devel: document expectations for HMP commands in the future

We no longer wish to have commands implemented in HMP only. All commands
should start with a QMP implementation and the HMP merely be a shim
around this. To reduce the burden of implementing QMP commands where
there is low expectation of machine usage, requirements for QAPI
modelling are relaxed provided the command is under the "x-" name
prefix.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

docs/devel/writing-monitor-commands.rst

@@ -11,6 +11,14 @@ For an in-depth introduction to the QAPI framework, please refer to
 docs/devel/qapi-code-gen.txt. For documentation about the QMP protocol,
 start with docs/interop/qmp-intro.txt.
 
+New commands may be implemented in QMP only.  New HMP commands should be
+implemented on top of QMP.  The typical HMP command wraps around an
+equivalent QMP command, but HMP convenience commands built from QMP
+building blocks are also fine.  The long term goal is to make all
+existing HMP commands conform to this, to fully isolate HMP from the
+internals of QEMU. Refer to the `Writing a debugging aid returning
+unstructured text`_ section for further guidance on commands that
+would have traditionally been HMP only.
 
 Overview
 --------