docs - clarify MetadataDbClient methods

[tmchartrand]

I was realizing recently I haven't used and don't fully understand the MetadataDbClient specific methods, and the docs weren't too much help. Would be great to clarify exactly what steps the register/deregister methods take, and what the assumptions are (ie presence of metadata json files). This could be in docstrings, readthedocs, or both. Thanks!