Unclear example on mutability

[mark-summerfield]

This is copied from rust-lang/book#102 (comment) as requested:

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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.

In the link I was given: http://rust-lang.github.io/book/variable-bindings.html#Mutable there is an updated explanation which explains this clearly.

[mark-summerfield]

I tried fixing this one: #33720