RFR (JAXP) 8179868: Java API Docs of javax.xml.transform.stax contains TODOs

[huizhe wang]

Hi,

Please review a fix for stax's package description. This is in a format 
similar to its SAX/DOM/Stream counterparts. For JDK 10, we could 
consider converting to package-info.java.

JBS: https://bugs.openjdk.java.net/browse/JDK-8179868
webrevs: http://cr.openjdk.java.net/~joehw/jdk9/8179868/webrev/

Thanks,
Joe