Summary

Add default methods for new-in-9 DocTreeVisitor methods.

Problem

@since and default methods were missed for several APIs introduced in JDK 9

Solution

Add the necessary @since tags @implSpec tags and default methods for those APIs that were missed in JDK 9.

Specification

For convenience the specification is provided here: http://cr.openjdk.java.net/~ksrini/8184683/specdiff.out/overview-summary.html