qapi: Document visit_type_any() issues with keyval input

It's already documented in keyval.c (commit 0ee9ae7), but visitor.h
can use a note, too.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1495471335-23707-3-git-send-email-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>

include/qapi/visitor.h

@@ -607,6 +607,10 @@ void visit_type_number(Visitor *v, const char *name, double *obj,
  * @obj must be non-NULL.  Input visitors set *@obj to the value;
  * other visitors will leave *@obj unchanged.  *@obj must be non-NULL
  * for output visitors.
+ *
+ * Note that some kinds of input can't express arbitrary QObject.
+ * E.g. the visitor returned by qobject_input_visitor_new_keyval()
+ * can't create numbers or booleans, only strings.
  */
 void visit_type_any(Visitor *v, const char *name, QObject **obj, Error **errp);