Document differences between infrastructure and "operator" client for test infrastructure

[metacosm]

The difference between both clients and why it might be interesting to configure them differently should be documented, in my opinion. Otherwise, LGTM.

Originally posted by @metacosm in #2764 (review)

[xstefank]

I would add that we should document the whole LocallyRunOperatorExtension and how to write test in JOSDK.