Use define-minor-mode and make a global minor mode. Define your key
bindings there.
Thanks define-minor-mode is an idea ... but IMHO wouldn't help when extending major-mode keybindings ...
> c) attach documentation to them
Documentation are bound to functions, not key bindings. (You can add
comments where you bind the keys, of course.)
actually according to `define-key' documentation the DEF part in a keymap can hold much more than just a command, such that it should be possible to associate meta informations to a key binding.