MLT query: missing documentation for `fail_on_unsupported_field`

[Mpdreamz]

https://github.com/elastic/elasticsearch/blob/master/core/src/main/java/org/elasticsearch/index/query/MoreLikeThisQueryBuilder.java#L798

fail_on_unsupported_field is not documented https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-mlt-query.html

[elasticmachine]

Pinging @elastic/es-search-aggs