Write all fields, terms and postings. This the "pull"
API, allowing you to iterate more than once over the
postings, somewhat analogous to using a DOM API to
traverse an XML tree.
Notes:
- You must compute index statistics,
including each Term's docFreq and totalTermFreq,
as well as the summary sumTotalTermFreq,
sumTotalDocFreq and docCount.
- You must skip terms that have no docs and
fields that have no terms, even though the provided
Fields API will expose them; this typically
requires lazily writing the field or term until
you've actually seen the first term or
document.
- The provided Fields instance is limited: you
cannot call any methods that return
statistics/counts; you cannot pass a non-null
live docs when pulling docs/positions enums.