Add XML documentation for Org.Json

The `Org.Json` namespace contains **108** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **176** to **108** (reduced by **68**). Related documentation PRs: [#387](https://github.com/dotnet/android-api-docs/pull/387), [#455](https://github.com/dotnet/android-api-docs/pull/455), [#495](https://github.com/dotnet/android-api-docs/pull/495), [#595](https://github.com/dotnet/android-api-docs/pull/595), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start with the documented files under docs\xml\Org.Json and docs\xml\ns-Org.Json.xml, then run the provided rg command to review the 108 placeholders. Add XML documentation for the affected Org.Json members, and consider the work complete when the scan finds no remaining 'To be added' entries outside docs\xml\index.xml.

Written by the indexing model from the issue text.