Clarify OAuth 2.0 Client Credentials flow

[rroblak]

In working on AM-3739 AccountStore Not Respected During OAuth2 Client Credentials Flow I found that I didn’t feel like the docs did as good of a job in describing how to use the Client Credentials as it could have.

This is the doc I’m referring to: https://docs.stormpath.com/rest/product-guide/latest/auth_n.html

In particular, I couldn’t find any documentation that clued me in to the fact that for Directories of type "Stormpath Cloud", client_id and client_secret map to the Account’s apiKey and secret. It would be helpful if this was pointed out explicitly, perhaps also with a bit of motivation around why an end user would want to use the Client Credentials flow in the first place.