Clarify handling of repeatability of LRO POST action

[mikekistler]

Clarify the case of a long running post action with an existing operation id, where the request body is the same (retry scenario) in the guidelines. I read the spec, that always a 202 or 400 needs to be returned 400 when the payload is generally broken OR the operation id exists AND the request body is different. no conflict and no OK response for the retry. We mention the part of existing operationid, maybe we should just add some more text for non native speakers.