Updated threadpool documentation to elaborate/clarify what the pools are for and their values
Updated threadpool documentation to elaborate/clarify what the pools are for and their values

Closes #8446

fubuki/elasticsearch,aparo/elasticsearch,fubuki/elasticsearch,fubuki/elasticsearch,fubuki/elasticsearch,aparo/elasticsearch,aparo/elasticsearch,fubuki/elasticsearch,aparo/elasticsearch,aparo/elasticsearch,fubuki/elasticsearch,aparo/elasticsearch