Guide: comments

I'm leaving off `rustdoc` usage because it won't work unless this is a `pub fn`, and I want to talk about public/private in the context of modules. I'm also not mentioning `//!` because it is exclusively used to provide the overview of a module.
(e.g. it can and will complain about the following:

```rust
fn foo() {
    let a = 0u;
    /// descriptive comment
    let b = 1u;
}
```

because doc comments can only be attached to items.)