Document support level and limitations of ElytronAuthenticator.

As part of the Security context propagation RFE EAP7-284 there is also provided a java.net.Authenticator implementation - the org.wildfly.security.auth.util.ElytronAuthenticator. Due to Java 8 design limitations this authenticator doesn't work correctly.

We have to clearly state the ElytronAuthenticator support level in 7.1 documentation. Could the PM confirm? (QE suggests unsupported or technical preview)

bilge What support level do you prefer?

The documentation should also contain a warning related to this class. sth. like "Don't use this on server and understand how the credentials caching can limit you when you use it in a standalone client JVM."

Full discussion about the topic is tracked in EAP7-284 and JBEAP-12340.