docs: document SyntaxViolation variants, remove bare URLs

[aatifsyed]

Fix these errors running cargo doc:

warning: this URL is not a hyperlink
  --> url/src/quirks.rs:68:5
   |
68 | /// https://url.spec.whatwg.org/#dom-url-domaintounicode
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use an automatic link instead: `<https://url.spec.whatwg.org/#dom-url-domaintounicode>`
   |
   = note: bare URLs are not automatically turned into clickable links
   = note: `#[warn(rustdoc::bare_urls)]` on by default

And have SyntaxViolation describe its variants in rustdocs

[codecov]

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  Coverage        ?   81.91%           
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  Files           ?       20           
  Lines           ?     3549           
  Branches        ?        0           
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  Hits            ?     2907           
  Misses          ?      642           
  Partials        ?        0           

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