qapi: Document introspection stability considerations

qapi: Document introspection stability considerations

We are not ready (and might never be ready) to declare
introspection stable between releases. Clients written to
control multiple versions of qemu, and desiring to know
whether a particular member is supported for a given
command, must be prepared to locate that member in spite
of qapi changes that may affect the member's location or
type within the overall object, even though such changes
did not break QMP wire back-compatibility.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1447264202-19554-1-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>