[Bug libstdc++/58341] Doc conflicts with standard on forbidden range of `result` in copy_backward()

[kristian.spangsege at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58341

--- Comment #2 from Kristian Spangsege <kristian.spangsege at gmail dot com> ---
And more importantly, here:
http://cplusplus.github.io/LWG/lwg-defects.html#1206