[CD.x] Update the synchronous replication behavior inside the section "Clustering Modes"

Synchronous replication processing runs on the same thread to the user request, but session replication is initiated after the completed response is sent back to the client in EAP 7. (Note that it is initiated before the completed response is sent back to the client in EAP 6. )

This behavior is not clearly stated in the current description in the documentation: https://access.redhat.com/documentation/en-us/red_hat_jboss_enterprise_application_platform/7.1/html/configuration_guide/configuring_high_availability#clustering_modes

Synchronous replication
With synchronous replication, the thread that handles the user request is blocked until replication has been successful. When the replication is successful, a response is sent back to the client, and only then is the thread is released.

I would suggest updating the above part to:

With synchronous replication, the replication processing runs on the same thread to the thread that handles the user request. Session replication is initiated after a completed response is sent back to the client, then the thread is released only after the replication is completed.