document FTS multi-partition relevance scoring impacts

All of the tf-idf scoring for document relevancy is done on a per-pindex basis, which means that documents across different partitions may have different scores (where you may expect them to be the same).

We need to very clearly document this behavior so that it does not catch people out by surprise.

Possibly the FAQ page - https://docs.couchbase.com/server/6.0/fts/fts-troubleshooting.html - would be a good place to put this.