Documentation - configure client thread pool max size on server side

Current documentation contains section about client side thread management [1].
RFE EAP7-619 introduces new way of global client thread size configuration.

We should document

• Existence of new attributes and their use
  • Attributes are used to configure client thread pool max size on server side, this thread pool is used when server connects to another server.
• Possible conflict with previous system property configuration
  • Please take a look on JBEAP-7754 which describes issue with system property configuration and configuration by model attribute.

Configuration related CLI commands

/subsystem=messaging-activemq:write-attribute(name=global-client-thread-pool-max-size, value=20)

/subsystem=messaging-activemq:write-attribute(name=global-client-scheduled-thread-pool-max-size,value=50)

[1] https://access.redhat.com/documentation/en/red-hat-jboss-enterprise-application-platform/7.0/single/configuring-messaging/#client-side_thread_management