Add XML documentation for Android.Text.Format

The `Android.Text.Format` namespace contains **82** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

```powershell
rg -n -i --glob 'Android.Text.Format/**' --glob 'ns-Android.Text.Format.xml' 'to be added' docs\xml
```

## Update log

- 2026-09-10: Refreshed the placeholder scan from **103** to **82** (reduced by **21**). Related documentation PRs: [#478](https://github.com/dotnet/android-api-docs/pull/478), [#488](https://github.com/dotnet/android-api-docs/pull/488), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start with the provided rg scan under docs\xml for Android.Text.Format/** and ns-Android.Text.Format.xml, excluding docs\xml\index.xml. Review the 82 matching “To be added” placeholders and replace them with XML documentation. Done means the placeholder scan no longer finds entries in this namespace.

Escrito por el modelo de indexación a partir del texto del issue.