* include/std/system_error (error_category, error_code)
	(error_condition): Improve docs.
	* libsupc++/exception: Add missing @addtogroup Doxygen command.
	* libsupc++/exception_ptr.h (exception_ptr): Link equality operators
	to class documentation. Suppress documentation for implementation
	details.
	* libsupc++/nested_exception.h (throw_with_nested, rethrow_if_nested):
	Suppress documentation for implementation details.

From-SVN: r270873