Fixed error in the documentation of scala.math.PartialOrdering

The documentation of the `scala.math.PartialOrdering` trait contains a very confusing since – from a mathematical perspective – wrong statement. In particular, it reads:

> A partial ordering is a binary relation on a type `T` that is also an equivalence relation on values of type `T`. This relation is exposed as the `lteq` method of the `PartialOrdering` trait.

This suggests that every partial ordering is itself an equivalence relation, and that the relation exposed as the lteq method is an equivalence relation. Clearly, this is not correct: if *R* is a partial ordering, then in general only the **intersection** of *R* and *R^(-1)* is an equivalence relation. I therefore propose to adjust the documentation.

Additionally, the existing documentation had some minor formatting inconsistencies, which have been fixed as well.