reference: clarify use declaration location

[matklad]

Reference implied that use declarations may appear only at the top of blocks and modules, but it is not the case, and the following is valid:

fn foo() {
    let x = 92;
    use baz::bar;
}

r? @steveklabnik

[steveklabnik]

@bors: r+ rollup

thanks!

[bors]

📌 Commit 397ab31 has been approved by steveklabnik

[matklad]

and the following is valid.

I was sure I had actually compiled it! But that is in fact invalid and fails with imports are not allowed after non-item statements E0154.

However the following is valid (definitely compiled and run this time!)

fn main() {
    fn foo() {}
    use std::io;
}

So documentation fix is actually correct.