Add XML documentation for Android.Views.TextService

The `Android.Views.TextService` namespace contains **117** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **125** to **117** (reduced by **8**). Related documentation PRs: [#360](https://github.com/dotnet/android-api-docs/pull/360), [#440](https://github.com/dotnet/android-api-docs/pull/440), [#547](https://github.com/dotnet/android-api-docs/pull/547), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Run the provided rg scan under docs/xml for Android.Views.TextService/** and ns-Android.Views.TextService.xml to list the 117 placeholders. Read the affected XML entries and existing documentation in that namespace to determine the appropriate descriptions. Done means the reported "To be added" placeholders are replaced with XML documentation and the scan confirms none remain.

Scritto dal modello di indicizzazione a partire dal testo della issue.