[Docs] Composite aggs docs talk about 3 choices but there are 4

[hendrikmuhs]

Small docs issue:

https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-composite-aggregation.html#_values_source

contains:

There are three different types of values source:

Scrolling down there are actually 4: terms, date_histogram, histogram and geotile_grid

[elasticmachine]

Pinging @elastic/es-docs (>docs)