Stefan Monnier <monnier@iro.umontreal.ca> writes (in gnu-emacs-sources):
>
> I'd point out that fill-nobreak-predicate is a hook in recent
> versions of Emacs

I notice the manual still only says nil or a function.  Perhaps
something like the following (new text for ease of reading, diff
below),


   The variable `fill-nobreak-predicate' is a hook (an abnormal hook,
*note Hooks::) specifying additional conditions where line-breaking is
not allowed.  Each function is called with no arguments and should
return a non-`nil' value if point is not a good place to break the
line.  Two standard functions you can use are
`fill-single-word-nobreak-p' (don't break after the first word of a
sentence or before the last) and `fill-french-nobreak-p' (don't break
after `(' or before `)', `:' or `?').


2005-08-27  Kevin Ryde  <user42@zip.com.au>

        * text.texi (Fill Commands): fill-nobreak-predicate is now a hook.