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.

mods/farming/nodes.lua

@@ -113,7 +113,8 @@ stairs.register_stair_and_slab(
 	{"farming_straw.png"},
 	"Straw Stair",
 	"Straw Slab",
-	default.node_sound_leaves_defaults()
+	default.node_sound_leaves_defaults(),
+	true
 )
 
 minetest.register_abm({