[Docs] Unit of measure for Tilemap dimensions

[B-Reif]

From the docs for Builder dimensions, it's not clear what units the dimensions are in. This could potentially be a number of tiles, a number of chunks, or even a number of pixels.

It would help to annotate this method with the unit it's expecting. Since this library expects a variety of different measures in various methods, it could also help to introduce newtypes to describe sizes and units of measure.

EDIT: From the source comments, it looks like it's measured in chunks.

[B-Reif]

Opened a small PR (#159) to update the method docs for now.

Using units of measure would cause breaking changes and would have wider implications.