bug#73098: setopt float warning unexpected

[Bug reports for GNU Emacs, the Swiss army knife of text editors]

> setopt will execute any custom-set form
                          ^^^^^^^^^^^^^^^
> associated with VARIABLE

1. "custom-set form" isn't mentioned, let alone
   defined, in the Elisp manual.  Should this
   say "any defcustom :set function" instead?

2. What about what I suppose you would call a
   "custom-init form", in other words, what the
   Elisp manual calls ":initialize function"?