Stairs: Add field to determine world-aligned textures (#2219)

Stairs: Add field to determine world-aligned textures (#2219)

All stair/slab nodes with parent nodes that are rotatable (wood and bricks)
are reverted to not having world-aligned textures, to fix the breakage of
rotated stair/slab appearence in worlds.

Update, and add missing documentation to, game_api.txt.