Add guidance for pageable post operations

There are situations where a query-like operation must accept inputs that are too complex or bulky to pass in the query string, and since GET cannot accept a request body, the operation instead uses the POST method. When the response to this method could be large, it should be pageable. But the pageable pattern is not designed for POST methods.

We should have a recommended pattern for pageable post operations. The guidance should address:

• are operations to get subsequent pages POST or GET?
• if POST, does the request body have to be supplied?
• if the request body is required for subsequent pages, does it have to be the same as on the original request? What happens if it isn't?

Review the existing pageable pattern in the API guidelines first, then compare it with the POST-operation scenario described here. Done means documented recommendations that answer whether follow-up pages use POST or GET, whether the body is required, and how differing bodies are handled.