update identifier naming warnings to give an example

This updates identifier warnings such as ``struct `foo_bar` should have a
camel case identifier`` to provide an example.

Closes #14738.
I think this can actually just be

```rust
buf.extend(words.chars().enumerate().map(...));
```
i.e. no `collect` or `as_slice`.

In fact, the whole function can probably be

```rust
s.split('_').flat_map(|word| word.chars().enumerate().map(...)).collect::<String>()
```

(Could you also rename `v` to `c`?)