Add XML documentation for Android.Transitions

The `Android.Transitions` namespace contains **175** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **189** to **175** (reduced by **14**). Related documentation PRs: [#423](https://github.com/dotnet/android-api-docs/pull/423), [#571](https://github.com/dotnet/android-api-docs/pull/571), [#594](https://github.com/dotnet/android-api-docs/pull/594), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the provided rg scan under docs\xml, focusing on Android.Transitions/** and ns-Android.Transitions.xml, then review the surrounding source documentation and related documentation PRs. Done means the 175 reported “To be added” placeholders in the Android.Transitions namespace have been replaced with accurate XML documentation and the scan no longer reports them.

Written by the indexing model from the issue text.