Tweak Fdocumentation's error for an undefined function

* src/doc.c (Fdocumentation): Restore the pre-25 behavior
of signalling a void-function error for an undefined function.
This seems cleaner than Emacs 25's "invalid-function: nil" error,
which was (probably) an unintended consequence of changes to
Findirect_function.

src/doc.c

@@ -336,6 +336,8 @@ string is passed through `substitute-command-keys'.  */)
     }
 
   fun = Findirect_function (function, Qnil);
+  if (NILP (fun))
+    xsignal1 (Qvoid_function, function);
   if (CONSP (fun) && EQ (XCAR (fun), Qmacro))
     fun = XCDR (fun);
   if (SUBRP (fun))