Add code example of smoothing model parameter for phrase suggester

[gskema]

https://www.elastic.co/guide/en/elasticsearch/reference/master/search-suggesters-phrase.html#_smoothing_models

This does not show what parameter should be used to configure the smoothing model. I assume it is "smoothing": .... This should be added in the documentation. A code example would be even better.

[jimczi]

cc @elastic/es-search-aggs