Every feature should be documented in the manual. If you find an area where the manual is "lacking," please (1) do a search of the Flyspray bug reports to see if it has been previously submitted as a bug, and (2) if not, submit it as a bug under the category "Manual."
To answer your question, "where can one find all the extra features Rockbox has and are not on the menus?" -- See Section 1.2 of the Manual, "Getting More help."
To answer your question, "who reads the manual?" -- Obviously, you should.