(Fminibuffer_completion_contents): New Lisp function

(Fminibuffer_completion_contents): New Lisp function
created from minibuffer_completion_contents.
(minibuffer_completion_contents): Remove.
(do_completion, Fminibuffer_complete_word)
(Fminibuffer_completion_help): Replace minibuffer_completion_contents
with Fminibuffer_completion_contents.
(syms_of_minibuf): Add Sminibuffer_completion_contents.
(Fdisplay_completion_list): Doc fix.
(display_completion_list_1): Use `nil' for second arg of
`Fdisplay_completion_list'.