lua_api.txt clarifications regarding mapgen object index IDs and handles.

[shacknetisp]

Currently there is no mention that these are different, so it took a while of trial and error before I discovered that minetest.get_decoration_id's return was actually different from minetest.register_decoration's return value.

[SmallJoker]

minetest.register_ore and minetest.register_biome "suffer" from the same issue. Please refer to the mapgen object-specific getter functions for all of them.

[shacknetisp]

I've generalized this more to differentiate between biome/decoration/etc. index IDs and their object handles.

[shacknetisp]

Outside the scope of this PR, but there are some tabulators here:

minetest/doc/lua_api.txt

Lines 2062 to 2067 in 8efa1de

	* `x` - The middle will be x pixels from all sides.
	* `x,y` - The middle will be x pixels from the horizontal and y from the vertical.
	* `x,y,x2,y2` - The middle will start at x,y, and end at x2, y2. Negative x2 and y2 values
	will be added to the width and height of the texture, allowing it to be used as the
	distance from the far end.
	* All numbers in middle are integers.

[paramat]

👍