Fix and clarify skybox texture order documentation

[GreenXenith]

The order of skybox textures is currently listed as Y+ (top), Y- (bottom), X- (west), X+ (east), Z+ (north), Z- (south). Testing with simple T/B/W/E/N/S textures and comparing with F5 proves this is clearly not the case.

The original Irrlicht CSkyBoxSceneNode defines Z- as the "front" and a direction order of left/right/front/back, making the plane order X+/X-/Z-/Z+. The documentation was erroneously based on the direction order rather than the plane order.

Also, Irrlicht (now Minetest) aligns the top/bottom textures with the right (East) face (─━┼─), where most skybox textures are aligned with the front face (─┿──). I have clarified this accordingly.

Most people have probably used the order N/S/E/W, which effectively rotates the skybox such that the North texture is on the East face, which works fine as long as the texture doesn't need to be cardinally-aligned.

Aside: Don't make PRs when sleepy :D