Rollup merge of #24149 - bombless:update-faq, r=steveklabnik

I think "let is used to introduce variables" is incorrent.
You can use
```rust
match (42, true) {
    (x, y) => { /* ... */ }
}
```
to replace
```rust
let x = 42;
let y = true;
```
so it's nothing special for `let`.