Synchronous Replication snippets

Please :
1. Expand the durability section in the KV doc, with a `DurabilityLevel.Majority` example, like the one at lines 98-112 of
https://github.com/couchbase/docs-sdk-dotnet/blob/release/3.0alpha/modules/howtos/pages/kv-operations.adoc
(Sorry, GitHub's line highlighting feature is switched off by its automatic AsciiDoc display).
2. Add a Sync Rep-style durability section to the Node.js Sub-Doc page, along the lines of
https://docs-staging.couchbase.com/dotnet-sdk/3.0/howtos/subdocument-operations.html#durability
https://github.com/couchbase/docs-sdk-dotnet/blob/release/3.0alpha/modules/howtos/pages/subdocument-operations.adoc