Add comment explaining why jdisc_core depends on xml-apis.
Comment is from the old vespa repo git log.
<name>${project.artifactId}</name>
<dependencies>
<dependency>
+ <!-- Newer version than the one in rt.jar, including the ElementTraversal class needed by Xerces (Aug. 2015)-->
<groupId>xml-apis</groupId>
<artifactId>xml-apis</artifactId>
<version>1.4.01</version>
