src/utilities.lisp (merge-lists): Make default tie-break sane, and document.

src/utilities.lisp (merge-lists): Make default tie-break sane, and document.

  * Make sure the candidate list given to the tie-breaking `pick'
    function is sorted according to the /first/ list each candidate
    appears in, rather than the last one, because `remove-duplicates' by
    default keeps the last-appearing element of each equivalence class,
    unless I give `:from-end t'.  So do that.

  * Describe the tie-breaking behaviour now that it's sane.  This is
    sort of important because it's used by the output machinery.