[PATCH] Fix -W* documentation, update to contrib/check_warning_flags.sh

[Ralf Wildenhues]

Ping <http://gcc.gnu.org/ml/gcc-patches/2008-02/msg00882.html>
The bits below still need approval.  Thanks FX for the quick Fortran
approval!

> contrib/ChangeLog:
> 2008-02-20  Ralf Wildenhues  <Ralf.Wildenhues@gmx.de>
> 
> 	* check_warning_flags.sh: Instead of invoke.texi, take the path
> 	to the doc directory as argument.  Check that warnings listed in
> 	'gcc --help' are accepted by the compiler, and listed in the
> 	manuals.
> 
> gcc/ChangeLog:
> 2008-02-20  Ralf Wildenhues  <Ralf.Wildenhues@gmx.de>
> 
> 	* c.opt (Wsynth): Deprecate.
> 	* doc/invoke.texi (Option Summary, Warning Options): Document
> 	-Wno-format-contains-nul.