libstdc++: Add [[nodiscard]] to chrono conversion functions

Also add doxygen comments.

libstdc++-v3/ChangeLog:

	* include/bits/chrono.h (duration_cast, floor, round, abs, ceil)
	(time_point_cast): Add [[nodiscard]] attribute and doxygen
	comments.
	(treat_as_floating_point): Add doxygen commen.