cleanup summery line docs

1. changes `# header` to `header`
2. changes `[link](url)` to:

```
[link]
... [link]: url
```

Fixes #16928
@huonw Basically the same as before (both not a link). I'll revert this change if you still care it.
Before(current): http://doc.rust-lang.org/nightly/rand/isaac/struct.IsaacRng.html
After this change: http://liigo.com/tmp/summery-line/rand/isaac/struct.IsaacRng.html