write more thorough API documentation in the code

[ctb]

With #621, our readthedocs doc site will once again contain documentation auto-extracted from the Python code. w00t!

However, our auto-extracted documentation is not that useful, especially in comparison to our API examples, which have been updated much more recently.

we should think about this as we move forward, since it's becoming clear that the Python API for sourmash is super-duper useful.

we also need to fix up our MinHash(...) class documentation, which doesn't get auto-extracted because its an extension module that doesn't get built on readthedocs (although with #616 maybe we can move these docs up to the Signature(...) class.

[luizirber]

I would say moving more of the API examples into docstrings? I added some docstrings at some point, but only in some methods.
https://github.com/dib-lab/sourmash/blob/ceb9b1751e339824614196135aa98843482518c2/sourmash/sbt.py#L105-L125

Benefit of using docstrings is that they are available in IPython (with obj.method?) or even plain Python (with help(obj.method))

[ctb]

I think the docstrings are more useful for being complete discussions of the arguments, while the examples are aimed at people interested in stealing code to use the functions. Different but overlapping audiences.