Add types and cleanup docs in scan.py

[alexfikl]

Similar to #648.

[alexfikl]

For some reason the types don't render in the docs (at least locally). There are no errors from sphinx, so I'll have to do some more digging :\

[alexfikl]

Managed to figure out what was going on in there :\

For future reference (if nothing else) the issue was with index_dtype: Any = <class 'numpy.int32'>. Basically sphinx would to a repr(np.int32) to make the whole signature into a string and then later on do a ast.parse to add back hyperlinks. The parse would then throw a SyntaxError on that part and abort the whole thing :cry:

[inducer]

Thanks!