documentPostings method
Filters the KeywordPostingsMap by document ids.
Returns a subset of the KeywordPostingsMap instance that only contains entries
where a document id in docIds
has a key in the entry's postings lists.
Implementation
KeywordPostingsMap documentPostings(Iterable<String> docIds) =>
KeywordPostingsMap.from(this)
..removeWhere((key, value) =>
value.keys.toSet().intersection(docIds.toSet()).isEmpty);