[FTS] Recommend changing the wording for default status of FTS service

The current documentation indicates 'Full text search is a service that is installed by default and no special installation is necessary. Just make sure you enable the service when you provision your Couchbase Server node. ' https://developer.couchbase.com/documentation/server/5.0/fts/full-text-intro.html

imo, it would be more clear to add something on these lines

FTS service needs to be added(checked) at the time a node is added to a cluster. On a new node setup this service is enabled by default, alongwith data, index, n1ql services.

Note: On Production systems we highly recommend separating out the FTS service to an independent set of node(s).

i.e. The existing wording works okay for a Dev setup, but not for a Production cluster expectations.