[7.0] Documentation - Messaging - configuration of client thread pool for MDBs

We need add note to Configuring Messaging guide - chapter 30.1. About the Integrated Artemis Resource Adapter - it should be located behind "Inbound Connections" paragraph:

"Each session of MDB is using thread from client thread pool to consume message from destination.
Thread pool maximum size is by default set to 8 * <number_of_cpu_cores>. This does not have suite use cases with many MDB sessions and can potentially lead to thread exhaustion and cause MDBs to be stuck waiting for another thread from the pool.

The maximum pool size of client thread pool can be increased by system property activemq.artemis.client.global.thread.pool.max.size. For example it can be set to 128 by adding -Dactivemq.artemis.client.global.thread.pool.max.size=128 system property to command line when starting the server."