Clarify the meaning of hir::Ty

[euclio]

In the Allocating and Working with Types section, it mentions that ty::Ty should not be confused with Ty in the HIR. How are they different?

[mark-i-m]

hir::Ty is more of a syntactic construct representing the syntax of a type. ty::Ty represents the semantics of a type, e.g. for type checking.

[eddyb]

Another way to think about it is every time you write a type, e.g. (), it gets its own ast::Ty and hir::Ty nodes (with IDs and source information), whereas the Ty is always exactly one (ty::Tup([])).

[mark-i-m]

fixed in #530