Document the COO serialization

#600 removed the `BQM.to_coo` and `BQM.from_coo` methods. We should therefore document and promote the `dimod.serialization.coo.dumps`, `coo.dump`, `coo.loads`, `coo.load` functions as an alternative.

Updates:
* #616 restored the methods, though with a deprecation note.
* #619 adds the functions to the documentation but does not improve the actual docstrings

Start by reviewing the documentation and docstrings for dimod.serialization.coo.dumps, coo.dump, coo.loads, and coo.load, along with the changes described in #619. Confirm how the deprecated BQM.to_coo and BQM.from_coo methods are presented. Done means the COO serialization functions are clearly documented and promoted as the alternative.

Written by the indexing model from the issue text.