qmp: document strict parsing

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

QMP/qmp-spec.txt

@@ -209,13 +209,27 @@ incompatible way are disabled by default and will be advertised by the
 capabilities array (section '2.2 Server Greeting'). Thus, Clients can check
 that array and enable the capabilities they support.
 
-Additionally, Clients must not assume any particular:
+The QMP Server performs a type check on the arguments to a command.  It
+generates an error if a value does not have the expected type for its
+key, or if it does not understand a key that the Client included.  The
+strictness of the Server catches wrong assumptions of Clients about
+the Server's schema.  Clients can assume that, when such validation
+errors occur, they will be reported before the command generated any
+side effect.
 
-- Size of json-objects or length of json-arrays
+However, Clients must not assume any particular:
+
+- Length of json-arrays
+- Size of json-objects; in particular, future versions of QEMU may add
+  new keys and Clients should be able to ignore them.
 - Order of json-object members or json-array elements
 - Amount of errors generated by a command, that is, new errors can be added
   to any existing command in newer versions of the Server
 
+Of course, the Server does guarantee to send valid JSON.  But apart from
+this, a Client should be "conservative in what they send, and liberal in
+what they accept".
+
 6. Downstream extension of QMP
 ------------------------------