RFR (JAXP) 8175830: Provide javadoc description for jdk.xml.dom module

[huizhe wang]

Hi,

Please review a module description for jdk.xml.dom.

JBS: https://bugs.openjdk.java.net/browse/JDK-8175830
webrev: http://cr.openjdk.java.net/~joehw/jdk9/8175830/webrev/

Thanks,
Joe