Document Windows file path handling in URL parsing (WHATWG URL #874)

## What information was incorrect, unhelpful, or incomplete?
The URL API documentation should be updated to reflect the
new Windows file path handling feature being added to the
WHATWG URL specification.

## What did you expect to see?
Documentation explaining that Windows drive letter patterns
(like `C:\folder\file.txt`) are automatically converted to
`file:///` URLs when parsed.

## Example
```javascript
new URL("C:\\folder\\file.txt")
// Returns: file:///C:/folder/file.txt
```
Implementation status

Implementation bugs have been filed:
- Chromium: https://issues.chromium.org/issues/437449112
- Gecko: https://webcompat.com/issues/170937
- WebKit: https://bugs.webkit.org/show_bug.cgi?id=297169
- Deno: https://github.com/denoland/deno/issues/30363
- Node.js: https://github.com/nodejs/node/issues/59415

Related links

- Spec PR: https://github.com/whatwg/url/pull/874
- Original issue: https://github.com/whatwg/url/issues/873
- Tests:
https://github.com/web-platform-tests/wpt/pull/53459

This documentation update should be made once the spec
change is merged and browsers begin implementation.