Review Check API documentation

[eric-murray]

Problem description

• the check API is not fully clear. One part seems to be about checking that a certain volume is still available (which has nothing to do with thresholds as such) an which is already covered by the /retrieve API, while another part seems to want to check the usage versus the thresholds somehow. In several descriptions a "requested threshold" is mentioned, while the threshold is not a parameter/entity in the request body.