RFR (JAXP) 8138721 : ElementTraversal: javadoc warning; also, hasFeature shall return true

[huizhe wang]

Hi,

Thanks Lance and Daniel for pointing out the javadoc warning! Here's the 
fix.  Also, added ElementTraversal to the DOMImplementation supported 
feature list.

JBS: https://bugs.openjdk.java.net/browse/JDK-8138721
http://cr.openjdk.java.net/~joehw/jdk9/8138721/webrev/

Thanks,
Joe