RFR 8079900: javadoc is missing for jdk.nashorn.api.tree package

[A. Sundararajan]

Please review fix for  
http://cr.openjdk.java.net/~sundar/8079900/webrev.00/ for 
https://bugs.openjdk.java.net/browse/JDK-8079900#comment-13640520

Note that jdk.nashorn.api.tree is part of nashorn API as specified by 
Nashorn Parser API JEP http://openjdk.java.net/jeps/236

Thanks,
-Sundar