Keywords supported by define-minor-mode

[Eli Zaretskii <eliz@gnu.org>]

I needed today to understand what exactly this does:

  (define-minor-mode midnight-mode
    "Non-nil means run `midnight-hook' at midnight."
    :global t
    :initialize #'custom-initialize-default
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

I was astonished to find out that this keyword is not documented
anywhere, neither in the doc string of define-minor-mode, nor in the
manual.  The full list of define-minor-mode's keywords that aren't
documented is this:

  :extra-args
  :set
  :initialize
  :type

Could someone please document these, or at least describe them here in
their own words (so that someone else could update the documentation
accordingly)?  We shouldn't leave such important functionality
undocumented for so long.

TIA