Some improvements to TileSetScenesCollectionSource docs

[KoBeWi]

Adds some clarifications to TileSetScenesCollectionSource description regarding how scenes are indexed. Also adds a code example how to get scene ID from a tile, because it's not obvious.

Closes #73645

[Mickeon]

Still insane to me that one would have to follow that relatively verbose example because the API is not exactly accessible.

[KoBeWi]

It can be reduced to single (less safe) line, but it's not exactly friendly:

tile_set.get_source(tile_map_layer.get_cell_source_id(cell)).get_scene_tile_scene(tile_map_layer.get_cell_alternative_tile(cell))

We could add a method similar to get_cell_tile_data(), which is only relevant to TileSetAtlasSource, but there is no high demand for it.

[Mickeon]

Strictly speaking the fact that it takes so many operations to get the "starting information" that you need is insane. It's not just for getting Scene Collections, it's for getting information on tiles in general (I speak by experience). I guess it's also a byproduct of cell data being split in 3 like this.

[akien-mga]

Thanks!