Add comment explaining why jdisc_core depends on xml-apis.
Add comment explaining why jdisc_core depends on xml-apis.

- Comment is from the old vespa repo git log.

vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa