quaternion: Document init_from_quaternion()

Reviewed-by: Robert Bragg <robert@linux.intel.com>

(cherry picked from commit 91c2a3580a6055fcb6902524d3e8354f27932e89)

cogl/cogl-quaternion.h

@@ -258,6 +258,13 @@ void
 cogl_quaternion_init_from_euler (CoglQuaternion *quaternion,
                                  const CoglEuler *euler);
 
+/**
+ * cogl_quaternion_init_from_quaternion:
+ * @quaternion: A #CoglQuaternion
+ * @src: A #CoglQuaternion with which to initialize @quaternion
+ *
+ * Since: 2.0
+ */
 void
 cogl_quaternion_init_from_quaternion (CoglQuaternion *quaternion,
                                       CoglQuaternion *src);