Missing docs for the `kuromoji_completion` filter

[pakio]

In the work of #81858 kuromoji_completion filter was added but the documentation was not provided along with the change. To use this feature, users somehow figure out this feature on their own at the moment which is not ideal.

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)

[PeteGillinElastic]

Team, please note that @pakio has kindly raised a PR for this: #117808.