Consider storage options and documentation for FTS storage

Currently, operator treats FTS as "stateless" though it does actually have state. The failure or elimination of sufficient FTS nodes will surely cause it to be unresponsive until the indexes are rematerialized.

Cannot change this without breaking existing clusters. Definition of done with this is to document that though FTS is technically stateless, recovery of it may take a large amount of time and thus it is recommended to do this with a PV. This should therefore be documentation.

This originally came up in review for the ephemeral cluster best practices: http://review.couchbase.org/c/couchbase-operator/+/139908