Document no-conflicts mode

There is a bit of discussion here:

https://github.com/couchbaselabs/couchbase-mobile-portal/blob/master/md-docs/_20/guides/couchbase-lite/native-api/conflict-resolution/swift.md

But I think it needs a more thorough explanation. Also, there there might need to be some upgrade-specific docs that cover cases where docs on sync gateway are in conflict, as described in https://github.com/couchbase/couchbase-lite-core/issues/431