Add XML documentation for Android.Views.TextClassifiers

The `Android.Views.TextClassifiers` namespace contains **590** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **1,240** to **590** (reduced by **650**). Related documentation PRs: [#360](https://github.com/dotnet/android-api-docs/pull/360), [#438](https://github.com/dotnet/android-api-docs/pull/438), [#644](https://github.com/dotnet/android-api-docs/pull/644), [#667](https://github.com/dotnet/android-api-docs/pull/667).

Run the provided rg command from the repository root to list the 590 matching placeholders under docs\xml, excluding docs\xml\index.xml. Review the matching Android.Views.TextClassifiers source XML documentation entries and replace the placeholders with accurate documentation; the work is done when the scan finds no remaining 'to be added' entries in that scope.

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