libstdc++: Fix documentation for return values of copy algos

libstdc++: Fix documentation for return values of copy algos

The doxygen comments for these algos all incorrectly claim to use
(first - last) as the difference from the start of the output range to
the return value. As reported on the mailing list by Johannes Choo, it
should be (last - first).

libstdc++-v3/ChangeLog:

	* include/bits/stl_algobase.h (copy, move, copy_backward)
	(move_backward): Fix documentation for returned iterator.