Add XML documentation for Android.Views.Accessibility

The `Android.Views.Accessibility` namespace contains **703** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **636** to **703** (increased by **67**). Related documentation PRs: [#373](https://github.com/dotnet/android-api-docs/pull/373), [#402](https://github.com/dotnet/android-api-docs/pull/402), [#519](https://github.com/dotnet/android-api-docs/pull/519), [#564](https://github.com/dotnet/android-api-docs/pull/564), [#583](https://github.com/dotnet/android-api-docs/pull/583), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the provided ripgrep scan in PowerShell and review the matching files under docs/xml for Android.Views.Accessibility and ns-Android.Views.Accessibility.xml, excluding docs/xml/index.xml. Replace the 703 `To be added` placeholders with XML documentation; the work is done when the scan no longer finds those placeholders.

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