(fn ...) - please fill it at the point of generation

[Drew Adams]

IIUC, it is make-docfile.c that adds the "(fn ..)" to a doc string, which
causes the displayed documentation to show the function's signature.

A while ago, I asked that this printed signature be wrapped. My motivation
was that a very long line can throw off window-fitting and frame-fitting
that might measure the longest line length, resulting in, say, wasted frame
space.

This was fixed - thanks.

However, I also recuperate doc strings using function `documentation' and
present them as completion candidates, and in this case the signature is
still one long line.

I would like the filling to be done at the point where the (fn...) is
generated, not just when it is displayed by `C-h f'. The entire doc string
is correctly within reasonable width limits, except for this one line, which
can get way out of hand.

Here, for instance is the (fn...) line for function `quail-define-package'
(the mailer will fill it, but imagine it unfilled):

(quail-define-package name language title &optional guidance docstring
translation-keys forget-last-selection deterministic kbd-translate
show-layout create-decode-map maximum-shortest overlay-plist
update-translation-function conversion-keys simple)

That's a line length of 250 characters!

AFAIK, the only purpose of this signature string is for documentation, that
is, for contexts in which a normal line length is expected. The fact that it
can be essentially limitless is, to me, a bug - it doesn't fit its use in a
doc string.

Could this please be fixed, so that functions such as `documentation' and
`documentation-property' return a doc string that has reasonable-length
lines? Thanks.