Customization docs plus some minor changes

A few minor changes, mostly the docs part.

I changed the M-> to M-< for emacs copy mode to be coherent with vi. However, to avoid unnecessary visual glitches, I'd prefer to have `send-keys L` before adding padding instead of needing to scroll back and forth to the beginning. Unfortunately `L` does not seem to have any emacs counterpart, nor any command name :(