It took me awhile to find it. Though here is the page that lists what would be considered excess baggage for 1Ps. http://www.united.com/page/article/0,6722,1034,00.html

I also can not find on UA where it says that 1P and up and F/C get the priority tags.

All this baggage info got a lot more complicated when they started charging everyone else for the first and second bags.