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.
Add detection of dead struct fields
Remove the dead code identified by the new lint