Particles: Make attached particle spawners respect the parent's yaw

Position, velocity and acceleration vectors of particles are rotated
by the yaw of the parent object so that they are truly relative to it.
Clarify new attached particle spawner behavior in lua_api.txt.

src/content_cao.h

@@ -136,6 +136,10 @@ public:
 	aabb3f *getSelectionBox();
 
 	v3f getPosition();
+	inline float getYaw() const
+	{
+		return m_yaw;
+	}
 
 	scene::ISceneNode *getSceneNode();