Guide: Standard input

I know you're trying to keep it simple until later but I think a few more details here would be helpful. I thought this was puzzling. Too few details and the user will modify something and confuse themselves without recourse.

> So Rust expects us to handle this case.

What you mean by this is unclear. Rust expected us to handle it; what if we don't? Will it crash? Will Rust refuse compilation?

Why tell it `ok()`? Why not just go directly to `expect()`. If `ok()` is to tell Rust everything is gonna be fine, when it fails then we lied (it wasn't `ok()`). How did `ok()` help?

Also, see below:
```rust
// I'll remove the error handling to see what happens...
// Should this compile because some case wasn't handled? Apparently it does...
let input = io::stdin().read_line(); // `Ok(test)` instead of `test`...
let input = io::stdin().read_line().ok(); // `Some(test)`...
// Rereads explanation and is more confused about `Ok` and `Some`
```

Here's a different type of explanation (hopefully sorta accurate):
> Rust will encapsulate the input as protection against possible errors. The user must remove this layer in order to use it. `ok()` transfers the data from a one wrapper to another wrapper (why? I don't know) so that errors can be handled properly. `expect()` removes the layer except during errors. During an error, it shows the message it was given.

Not advocating my wording. I just wanted to give a possible example so it wasn't just a complaint.

[EDIT] Moved comment to diff instead of on file