Fix PartialEq documentation with regards to floats

It is in fact the case that `NaN != NaN`. The true relations for
compareQuietNotEqual are LT, GT *and* UN.

I also rephrased the docs for PartialOrd since floats are not the only
types which are not totally ordered.
Clarify restrictions on ne

I can't think of any sane cases where this restriction would not hold,
and the standard library seems to assume it pretty much everywhere.
auto merge of #14733 : sfackler/rust/partial-eq-nan-docs, r=alexcrichton

It is in fact the case that `NaN != NaN`. The true relations for
compareQuietNotEqual are LT, GT *and* UN.

I also rephrased the docs for PartialOrd since floats are not the only
types which are not totally ordered.