Add `pruneAge` to the REST syntax documentation

Beth Favini 

We can set the audit log rotation setup using a REST call.

The documentation mentions rotateInterval and rotateSize.

We also have pruneAge , added in 7.2.4, which needs to be added when 7.2.4 goes live.

https://docs.couchbase.com/server/current/rest-api/rest-auditing.html#curl-syntax