Add documentation for changing authentication type of session.

[barestides]

It took a fair amount of digging to find that setting the "PreferredAuthentications" option for creating a session allows you to change the auth type. It was defaulting to Kerberos for me.

Scroll down to "User Authentication Methods" for all of the options.
https://epaul.github.io/jsch-documentation/javadoc/com/jcraft/jsch/JSch.html#setConfig-java.lang.String-java.lang.String-