Add ICouchbaseCollection.TryGetAsync documentation to code docs

The TryGetAsync method is a newer extension method which has performance benefits over GetAsync because the status only is passed back and no exception is thrown if KeyNotFound is returned by the server.

The API docs do have documentation, but not the main SDK CRUD docs. Adding this will help customers and users resolve performance issues when expecting a KeyNotFound to be encountered and gives SDK3 more parity with SDK2 behavior that is expected in this specific case.

Note that there are threads on this subject in forums and SO.

https://www.couchbase.com/forums/t/c-avoid-documentnotfoundexception/34633