Add XML documentation for Android.Text.Util

The `Android.Text.Util` namespace contains **85** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **88** to **85** (reduced by **3**). Related documentation PRs: [#544](https://github.com/dotnet/android-api-docs/pull/544), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the provided rg command in docs\xml to locate the 85 `To be added` placeholders under Android.Text.Util and review the surrounding source XML documentation. Replace the placeholders with appropriate documentation for the affected APIs, then rerun the scan; done means no scoped matches remain, excluding docs\xml\index.xml.

Written by the indexing model from the issue text.