cleanup summery line docs

1. changes `# header` to `header`
2. changes `[link](url)` to:

```
[link]
... [link]: url
```

Fixes #16928
@alexcrichton No, rustdoc do render this as a normal link (without [ and ])

online render:
http://liigo.com/tmp/summery-line/core/cmp/trait.Ord.html