No, there nothing in the manual about it but it seems to be a "new issue" that doesn't need to be an issue at all. If it's used by Garrett and is on the plugs from the factory then why wouldn't you want to re-apply it when it's gone from the plug? Yes, it needs to be applied from time to time like changing the oil in your car. Just like the manual doesn't say you'll need to replace the O ring from time to time cause it will wear out (believe it or not) just like tires on your car do. So someday you'll have to do that too. Just remember to use it lightly and make sure it's user friendly on the rubber O rings. Cheaper lubes might cause the ring to expand to much making it loss with it's ID (inner diameter) not seating correctly. Always check for this, make sure the O ring is "set" correctly in place and a tiny bit of quality lube to relieve any friction while you push it in place and tighten the plug. This is the ONLY seal that keeps the water out.
So if you don't want to replace the tiny bit of lube that was on it when new then don't. But then when the O ring dries out and gets damaged and causes a leak requiring repairs and time lost in the field you can't complain. It makes me wonder why you'll cover a coil, why? Cause they'll wear out, right? Shouldn't they have made the coil bottom thicker and therefore not needing a coil cover? So it just makes sense that O rings will wear out too. They don't make O ring covers but they do make the lubes to keep them good, so why not lube them?
I hope Garrett revises the manual and advises this as routine maintenance and starts to include a tube of lube in all there new units and makes it available to all owners as well. It will save a lot of future headaches. Somebody told me long ago "That an ounce of prevention is worth a pound of cure" and this has been very true to me in the past.
But in the end it's the owners decision if they want to do this with there machines, I'm doing to cause I know it's best for my AT Pro and helps it remain waterproof.
Happy Hunting and Trails to All of You. Woodstock