Follow-up I7d4bb90: Message tweak + add code comment

When I was saying it in my head, "No description available."
sounded better to me than "Description not available", although
perhaps that's just me.

Add a code comment to Parser::addTrackingCategory telling people
to register their tracking categories.

Change-Id: I17bb0cf4b3118dda8647e0607588685c2b5cdb86