Rewrote CustomDiagnosticTask#getComment() to allow selfdiagnose.xml to override the task comment

Rewrote CustomDiagnosticTask#getComment() to allow selfdiagnose.xml to override the task comment, using the referred task as a fallback.

Previously, the referred task was the primary comment source. This resulted in the use of multiple locations for the values in the 'left column' of the selfdiagnose page.