Document the valid range of tile coordinates in TileMapLayer

[timothyqiu]

As mentioned in RocketChat, TileMapLayer converts cell coordinates to 16-bit when serializing data.

godot/scene/2d/tile_map_layer.cpp

Lines 2867 to 2869 in f418603

	// Store position in TileMap.
	encode_uint16((int16_t)(E.key.x), &cell_data_ptr[0]);
	encode_uint16((int16_t)(E.key.y), &cell_data_ptr[2]);

This behavior dates back to the Godot Is Open Source commit :) Although it is quite rare to use extremely large tile maps, it is perfectly valid for users to create one in the editor. However, the result of doing so may be unexpected.

So I think it's better to document it and acknowledge it as intended rather than as a bug.

[Repiteo]

Thanks!