cleanup summery line docs

1. changes `# header` to `header`
2. changes `[link](url)` to:

```
[link]
... [link]: url
```

Fixes #16928
By splitting up the contents of the link, this actually cause rustdoc to render summaries as just "[total order]" without any link because the actual markdown link is far down below. For now this may not be desirable.