Clarify what is returned by TileSet.get_source(source_id)

**Your Godot version:**
4.4beta1

**Issue description:**
While trying to get the custom data directly from a TileSet as opposed to the TileMapLayer, I wasn't able to find anyway to get a reference to TileSetAtlasSource which held the get_tile_data method. It took me some time and googling for it to finally click that TileSet.get_source doesn't actually return TileSetSource but either TileSetAtlasSource or TileSetScenesCollectionSource.

Even if it is technically returning a TileSetSource, I think it would be worth clarifying in the notes for TileSet.get_source what exactly is we should be expecting to be returned.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/latest/classes/class_tileset.html#class-tileset-method-get-source