Add XML documentation for Android.Widget.PhotoPicker

The `Android.Widget.PhotoPicker` namespace contains **47** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **101** to **47** (reduced by **54**). Related documentation PRs: [#433](https://github.com/dotnet/android-api-docs/pull/433), [#560](https://github.com/dotnet/android-api-docs/pull/560), [#629](https://github.com/dotnet/android-api-docs/pull/629), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Run the provided rg command against docs\xml to list the 47 remaining placeholders under Android.Widget.PhotoPicker, then inspect the matching XML entries and nearby documentation for context. Replace each placeholder with accurate API documentation and rerun the scan; done means no matching placeholders remain in that namespace and the XML stays valid.

Written by the indexing model from the issue text.