Unclear example on mutability

[mark-summerfield]

In https://doc.rust-lang.org/book/mutability.html at the end of the Mutability section it has these examples:

let (mut x, y) = (5, 6);
fn foo(mut x: i32) {

I think there should be some following text, e.g.

Note that here, x is mutable, but y is not.

[peschkaj]

Howdy @mark-summerfield, this repository is for the second edition of the book. Does the new section on [mutable bindings](http://rust-lang.github.io/book/variable-bindings.html#Mutable bindings) clear things up for you?

Could also file this issue against rust-lang/rust?

[steveklabnik]

Yes, please do!