Function reference signatures to `var: Type` ordering

[vpipkt]

@vpipkt Sometime before 1.0, we should change these signatures to use the var: Type ordering, as that's what type hints in Python 3.7 use (along with Scala).

That said, really want to get this stuff generated by Sphinx.

Originally posted by @metasim in https://github.com/locationtech/rasterframes/pull/271/files