Add XML documentation for Android.Speech.Tts

The `Android.Speech.Tts` namespace contains **82** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **90** to **82** (reduced by **8**). Related documentation PRs: [#550](https://github.com/dotnet/android-api-docs/pull/550), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Run the provided rg command against docs\xml to identify the 82 placeholders under Android.Speech.Tts, excluding docs\xml\index.xml. Read the affected namespace and type XML files and the corresponding Android Speech TTS API references to determine each member's description. Done means the targeted placeholders have been replaced with accurate documentation and the scan no longer reports them.

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