Document limitation of supported API within public org.infinispan module

Until https://issues.redhat.com/browse/JBEAP-26173 is merged, we need to document that usage of "internal" infinispan interfaces/classes, while possible to do so via the public org.infinispan module, is not supported.
Internal APIs include publicly accessible interfaces/classes residing outside of the following packages:

• org.infinispan
• org.infinispan.manager
• org.infinispan.configuration.cache
• org.infinispan.configuration.global
• org.infinispan.distribution
• org.infinispan.notifications
• org.infinispan.notifications.cachelistener.event
• org.infinispan.notifications.cachemanagerlistener.event
• org.infinispan.remoting.transport
• org.infinispan.util.concurrent