Update docs

docs/lua_api.txt

@@ -246,6 +246,12 @@ Matrix4x4
 	**set_z** (m, z)
 		Sets the z axis of the matrix *m*.
 
+	**rotation** (m) : Quaternion
+		Returns the rotation portion of the matrix *m*.
+
+	**set_rotation** (m, r)
+		Sets the rotation portion of the matrix *m*.
+
 	**translation** (m, a) : Vector3
 		Returns the translation portion of the matrix *m*.