Explicitly point out port 8091 has to enabled when using Username and Password fields

In current documentation https://docs.couchbase.com/server/6.5/manage/manage-xdcr/enable-full-secure-replication.html#specify-full-xdcr-security-with-credentials,  it's worth pointing out that the port `8091` has to be enabled when Username and Password fields are used while attempting to add remote Cluster Reference with full encryption.
If `8091` port is disabled, then the Client Certificate and its key has to be provided along with not providing the Username and Password in their respective fields as mentioned in https://docs.couchbase.com/server/6.5/manage/manage-xdcr/enable-full-secure-replication.html#specify-full-xdcr-security-with-certificates.