Rollup merge of #31863 - matklad:clarify-reference, r=steveklabnik

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:

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

r? @steveklabnik

diff --cc src/doc/reference.md
Simple merge