Improve enum_param

Add an argument to pass the result param to the caller instead of
having the caller have to pick it up from the builder.
Improve docs for node, clock and monitor
Pass spa_pod everywhere instead of spa_pod_object.
Pass result argument to spa_pod_filter to make things a little
nicer.

spa/tests/test-perf.c

@@ -388,7 +388,7 @@ static int make_nodes(struct data *data)
 static int negotiate_formats(struct data *data)
 {
 	int res;
-	struct spa_pod_object *format;
+	struct spa_pod *format;
 	struct spa_pod_builder b = { 0 };
 	uint8_t buffer[256];