My son has all sorts of food allergies...and I know we've flown with our EpiPen before...I just don't remember if I needed any specific documentation to bring it on board.
In the main disABILITIES Forum one of the first threads is the disABILITIES FAQs. On Post #15 we have a lot of information concerning air travel, including links to the TSA rules about medications. Also, Post #3 has a dietary section which will also help. To get directly to the disABILITIES FAQs from here just click on the link in my signature.
I carry an EpiPen and my DH carries needles for his insulin. We put these in our carry on bags and never are we questioned about them by airport security. We've never even had to take them out of the bags for inspection.