added documentation for the function oauth2_auth_url

[kritisingh1]

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Description

Documentation for the function oauth2_auth_url in ohapi/api.py

Related Issue

Related to #5

Example

[mcescalante]

This looks okay to me for now. I think that in time, as we get more stuff documented, this may want to actually move to inline function docs and we will use some kind of generator to generate docs (such as Sphinx), but this will be a good start 😄