Document OpenActiveOpenIdTestClient, recordWithIntercept, logWithIntercept in openactive-openid-client package

[lukehesluke]

The OpenActiveOpenIdTestClient and recordWithIntercept and logWithIntercept functions are used in other packages and so should be documented as part of the interface of Test Suite's openid-related packages.

The latter two are already somewhat documented. So, for them, it's just a case of checking that this is sufficient to explain their purposes

Spawned from #633

See this PR for an example of how the OpenActiveTestAuthKeyManager class has been documented: #603