RFR [16/java.xml] 8249643: Clarify DOM documentation

[Joe Wang]

Hello,

Please review a doc clarification for the org.w3c.dom package.

JBS: https://bugs.openjdk.java.net/browse/JDK-8249643
CSR: https://bugs.openjdk.java.net/browse/JDK-8249904

webrev: http://cr.openjdk.java.net/~joehw/jdk16/8249643/webrev_02/

Thanks,
Joe