Allow to define MLT _minDocFreq_ and _minTermFreq_ for _KNearestNeighborClassifier_ to tweak recall of _MoreLikeThis_ queries.
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Migrated from [LUCENE-5348](https://issues.apache.org/jira/browse/…
1. Add the ability to specify a proportion to `minDocs`, where this means "keep only terms appearing in this proportion of documents".
2. Add the ability to specify a proportion to `minTerms`, whe…