gcc/

	* c.opt (Wsynth): Deprecate.
	* doc/invoke.texi (Option Summary, Warning Options): Document
	-Wno-format-contains-nul.

gcc/fortran/

	* invoke.texi (Error and Warning Options): Document
	-Wline-truncation.

contrib/

	* 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.

From-SVN: r133059