Documentation example for InvariantType is strange

[cgaebel]

http://doc.rust-lang.org/std/kinds/marker/struct.InvariantType.html

It starts talking about cell for some reason. I think this was copy + pasted from InvariantLifetime and never updated.

[cgaebel]

cc @steveklabnik

[huonw]

InvariantLifetime doesn't have this example (atm), and, the example only makes sense with InvariantType since there's no lifetimes involved in the definition of the Cell type. I do think the example could probably have a little more description, but I don't think it's wrong.

For reference, it currently says:

/// The Cell type is an example which uses unsafe code to achieve
/// "interior" mutability:
///
/// ```
/// struct Cell<T> { value: T }
/// ```
///
/// The type system would infer that `value` is only read here and
/// never written, but in fact `Cell` uses unsafe code to achieve
/// interior mutability.