cleanup summery line docs

1. changes `# header` to `header`
2. changes `[link](url)` to:

```
[link]
... [link]: url
```

Fixes #16928
I was just clarifying what @alexcrichton meant, fwiw; in any case, it is not displayed like that in nightly: http://doc.rust-lang.org/nightly/std/cmp/#traits