Fixed mathematically wrong statement in the documentation of scala.math.PartialOrdering.

- Previous (misleading) documentation suggested that a partial ordering *is* an equivalence relation. This has now been clarified.
- Existing documentation on properties of partial orderings (reflexive, transitive, antisymmetric) had several formatting inconsistencies that have now been remove.
Merge pull request #3892 from misberner/docfix-PartialOrdering

Fixed error in the documentation of scala.math.PartialOrdering