Additions to the What's New document for FTS changes

There are some changes in FTS that should be mentioned in the What's New document:

https://docs-staging.couchbase.com/server/7.1/introduction/whats-new.html

Add details for the following in the FTS section:

https://issues.couchbase.com/browse/MB-47441 (Increase to the supported number of FTS indexes)

https://issues.couchbase.com/browse/MB-26024 (Rebalance optimisations).

Read up here:

https://docs.google.com/document/d/14ERNLgnSgeWmfEGp8rbVsQTR6_vJkkxsJM1sq-9wCjE/edit#heading=h.x2v8edv1h5x1