Lots of info floating around on pitching temps... I pitch at 70f. Pitching higher (I wouldn't go above 85 myself) would actually get the yeast to thriving faster but is reportedly responsible for off flavors in the beer. This combined with the fact that my fermenters usually run at 68-70 is why I chose 70 as my pitching temp.
The 90 minutes your kit refers to is an arbitrary number... You want to get it cooled off as quickly as possible so there is less chance of infection of the mort.... eerrrrr Wort
. At boiling temps, the typical infectious germs / bacteria that enter the wort are killed off. When you pitch your yeast, the yeast is very strong and will most likely kill off these invaders therefore less chance of infection after pitching. Between these times, you beer is very susceptible to infection.
The 84f you pitched at should be ok. Welcome to the addictive hobby of home brewing!