Add XML documentation for Android.Speech

The `Android.Speech` namespace contains **311** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **333** to **311** (reduced by **22**). Related documentation PRs: [#411](https://github.com/dotnet/android-api-docs/pull/411), [#475](https://github.com/dotnet/android-api-docs/pull/475), [#639](https://github.com/dotnet/android-api-docs/pull/639), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start with the `docs\xml` paths matched by the provided `rg` command, especially the `Android.Speech/**` files and `ns-Android.Speech.xml`. Review the existing XML documentation patterns and replace the 311 `To be added` placeholders with API descriptions. Re-run the same scan to confirm the targeted placeholders have been addressed.

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