It's a bolt that holds the detent spring in place that encourages the wheel to remain in the straight position and not wobble like it would tend to do otherwise. With the mower deck off the ground, pivot the wheel by hand from left to right, you'll feel the detent engage at the 12 o'clock position.
It may not be in the manual, buts it's in the factory parts diagram the dealer has. You can register on Messicks web site and see the parts diagram there.
It is in my manual: page 19. I have a F2100 It is a set bolt and is supposed to be adjusted when the front wheel starts to shimmy. You adjust the bolt to 7 mm from the base of the prominence to the bottom of the bolt head. I have been mowing with my deck for 29 years and have yet to have to adjust it.