RFR(jdk10/jaxp) 8186675: Javadoc of SAXSource contains implementation detail

[huizhe wang]

Hi,

Removing an implementation detail from the javadoc of SAXSource, please 
review:

JBS: https://bugs.openjdk.java.net/browse/JDK-8186675
webrev: http://cr.openjdk.java.net/~joehw/jdk10/8186675/webrev/

Thanks,
Joe