Revise "What's New in Version 7.6" Query section for Sequential Scans.

In https://docs.couchbase.com/server/current/introduction/whats-new.html#query-service for the bullet point:

"The Query Service adds support for sequential scans, which enables querying without an index."

We need to add that this is optional, controlled by RBAC.  Something like:

"The Query Service adds support for sequential scans, controlled by RBAC, which enables querying without an index."