Currently, the DocsEnum api assumes you might consumes freqs at any time.
Additionally the DocsAndPositionsEnum api assumes you might consume a payload at any time.
High level things such as queries know what kinds of data they need from the index up-front,
and the current APIs are limiting to codecs (other than Standard, which has these intertwined).
So, we either need DocsAndFreqsEnum, DocsPositionsAndPayloadsEnum, or at least booleans
in the methods that create these to specify whether you want freqs or payloads.
we did this for freqs in the bulkpostings API, which is good, but these DocsEnum apis
are also new in 4.0 and there's no reason to introduce non-performant APIs.
additionally when/if we add payloads to the bulkpostings API, we should make sure we keep
the same trend and require you to specify you want payloads or not up-front.