Add the documentation for the OAuth2 authentication service

[weierophinney]

We miss the documentation for the OAuth2 service, if present in Apigility

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Originally posted by @ezimuel at zfcampus/zf-apigility-documentation#12

[weierophinney]

Any work being done on this? We are running into this issue currently.

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Originally posted by @danieljin at zfcampus/zf-apigility-documentation#12 (comment)