Improve API docs for <chrono> and <ratio>

Improve API docs for <chrono> and <ratio>

	* doc/doxygen/doxygroups.cc (std::literals): Add documentation for
	inline namespace.
	* include/std/chrono: Improve docs.
	* include/std/ratio: Do not document implementation details.
	* testsuite/20_util/ratio/cons/cons_overflow_neg.cc: Adjust dg-error
	line numbers.
	* testsuite/20_util/ratio/operations/ops_overflow_neg.cc: Likewise.

From-SVN: r270988