Improve documentation to methods

[mcara]

Documentation for assign_bestrefs https://hst-crds.stsci.edu/static/users_guide/library_use.html#crds-assign-bestrefs should be improved and describe expected argument type and what arguments do. I came across a case when user was setting filepaths='ie6d07ujq_flc.fits' (as an example) and then CRDS erred with messages that it cannot find files i, e, 6, d, ...

It would be great if CRDS documentation could follow style guide https://github.com/spacetelescope/style-guides/blob/06111fd0e4054f7835a6cd83dec3ce9049b3b970/guides/python.md#docstrings. Other functions seems to be documented slightly better but each has its own style.