qapi: Belatedly update CompatPolicy documentation for unstable

Commit 57df0dff1a1 (qapi: Extend -compat to set policy for unstable
interfaces) neglected to update the "Limitation" paragraph to mention
feature 'unstable' in addition to feature 'deprecated'.  Do that now.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20231009110449.4015601-1-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>

qapi/compat.json

@@ -43,8 +43,8 @@
 # This is intended for testing users of the management interfaces.
 #
 # Limitation: covers only syntactic aspects of QMP, i.e. stuff tagged
-# with feature 'deprecated'.  We may want to extend it to cover
-# semantic aspects and CLI.
+# with feature 'deprecated' or 'unstable'.  We may want to extend it
+# to cover semantic aspects and CLI.
 #
 # Limitation: deprecated-output policy @hide is not implemented for
 # enumeration values.  They behave the same as with policy @accept.