qga schema: document generic QERR_UNSUPPORTED

Part of the wording was shamelessly stolen from Michael Roth's email.
Suggested-by: NMichael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: NLaszlo Ersek <lersek@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NMichael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

qga/qapi-schema.json

 # *-*- Mode: Python -*-*
 
+##
+#
+# General note concerning the use of guest agent interfaces:
+#
+# "unsupported" is a higher-level error than the errors that individual
+# commands might document. The caller should always be prepared to receive
+# QERR_UNSUPPORTED, even if the given command doesn't specify it, or doesn't
+# document any failure mode at all.
+#
+##
+
 ##
 #
 # Echo back a unique integer value, and prepend to response a