Documentation - add info how to setup client side thread pool on server and clients

Chapter "3.20. Thread Management" in https://access.stage.redhat.com/documentation/en/red-hat-jboss-enterprise-application-platform/version-7.0/configuring-messaging-applications/#server-side_thread_management

Problem is that "Client-Side Thread Management" is not just something what is on JMS clients, this also applies for Artemis server which is also using threads from this thread pool.

Server is using those thread for operations on cluster-connection, in-vm connectors and MDB sessions.

Default max size of this pool is 8 * (number of cpu cores).

We should provide information how to set max size of this thread pool - there is system property -Dactivemq.artemis.client.global.thread.pool.max.size=<value>