bug#17250: 24.3; find-variable gives search-failed error message

[Eli Zaretskii <eliz@gnu.org>]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Wed, 18 Apr 2018 00:58:39 +0200
> Cc: 17250@debbugs.gnu.org
> 
> But there are several places that use this function without checking the
> return value.  Are they relying on this function to bug out with a
> cryptic error, or are they all confident that the file name can be
> found?
> 
> So I haven't applied it yet.  Perhaps somebody else will weigh in...

I'm not weighing in ;-)

>  (defun help-C-file-name (subr-or-var kind)
>    "Return the name of the C file where SUBR-OR-VAR is defined.
> -KIND should be `var' for a variable or `subr' for a subroutine."
> +KIND should be `var' for a variable or `subr' for a subroutine.
> +If we can't find the file name, nil is returned."
                                   ^^^^^^^^^^^^^^^
"return nil".  We try to avoid passive tense.