docs: clarify `PhantomData` vs "phantom types" statement

[alevy]

In the documentation for PhantomData, it states that: "Though they both have scary names, PhantomData and "phantom types" are unrelated."

This seems to not be an accurate description of the relationship between the PhantomData marker and phantom types, the concept. They are related, just not the same thing.

/cc @steveklabnik