Fix inconsistent docstrings

[nwunderly]

Some documentation in oauth.py and client.py is documented using the wrong docstring style (generic RST format, we want NumPy style).

Need documentation for these methods reformatted. Bonus points for any additional improvements to documentation, like rewording or fixing formatting mistakes elsewhere.

[akhilsharmaa]

Hey, I would like to take this issue for HacktoberFest.