PartialOrd documentation is misleading

[stephan-tolksdorf]

The documentation for cmp::PartialOrd states that "if a < b then !(a > b) and vice versa", but the "vice versa" is somewhat misleading here since !(a > b) doesn't imply a < b if e.g. either of the values is a NaN float. The author probably wanted to say that a > b also implies !(a < b).