I know that the Rockbox manual can seem initimating at first, especially if you're not a gadget manual geek. My best recommendation--just skim the manual to find the information most pertinent to you (such as, the sections on the navigation and selection of music, the selection of display screen "themes," etc.), read that, and flag other interesting sections to come back to later. But keep in mind that even skimming the Rockbox manual will be more than reading the instructions for a new electric can opener--just be patient.
Note that for many people, many sections of the Rockbox manual will never come into play--don't let that intimidate you, and just pass those sections by. But it's there, if you're the type who likes to make the most of the firmware and to tweek.