Document clustering limitation on Azure VMSS offering

Azure VMSS just kills a EAP VM. There is no graceful shutdown. This puts limitations to clustering subsystem. Server-side state for HA should be externalized to 3rd party service (i.e. RHDG). Currently, http sessions externalization is supported, but it is not supported for Stateful Session bean[1].

This should be added into 2.2. Unsupported Features / Virtual Machine Scale Set in "Using JBoss EAP in Microsoft Azure" guide [2].

[1] https://issues.redhat.com/browse/EAP7-1807?focusedCommentId=19744750&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-19744750
[2] https://access.redhat.com/documentation/en-us/red_hat_jboss_enterprise_application_platform/7.4/html-single/using_jboss_eap_in_microsoft_azure/index#unsupported_features