Add documentation for extension/datasets.pyi

[Miking98]

Would be great to add documentation / type checking for the functions in extension/datasets.pyi since its not immediately obvious what these methods do from the code.

[Miking98]

Also, can we rename Dictionary to be Array or something -- the current name is confusing given its accessed via an integer index into a list of values.

[EthanSteinberg]

The key part of Dictionary is that it also supports fast inverse lookups. I think using the name Array would mask that.

[EthanSteinberg]

We really need python/mypy#13284 to do this right.