zshrc: Move more commented-out code to skel/.zshrc

zshrc: Move more commented-out code to skel/.zshrc

This time: $cdpath and $MANWIDTH

All commented-out examples should go into the skel file, because users
should make their specific changes in ~/.zshrc.pre and ~/.zshrc.local.
Having examples in the global rc file would encourage to make changes
there. Besides, commented-out examples make the setup harder to read and
to maintain.