Add couchbase cluster compatibility to documentation

We should probably specify for this given major.minor of the SDK, one of three things for Couchbase Cluster releases:

• unsupported
• supported
• supports all features

These might be an 'x', '—' and "✓" in a table, or whatever Amy comes up with.

This is, in part, planning for 3.0 including beta.

This should be based on the work done in JCBC-438, so it's blocked by that issue.