Add XML documentation for Javax.Xml.Datatype

The `Javax.Xml.Datatype` namespace contains **15** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

```powershell
rg -n -i --glob 'Javax.Xml.Datatype/**' --glob 'ns-Javax.Xml.Datatype.xml' 'to be added' docs\xml
```

## Update log

- 2026-09-10: Refreshed the placeholder scan from **50** to **15** (reduced by **35**). Related documentation PRs: [#442](https://github.com/dotnet/android-api-docs/pull/442), [#501](https://github.com/dotnet/android-api-docs/pull/501), [#511](https://github.com/dotnet/android-api-docs/pull/511), [#633](https://github.com/dotnet/android-api-docs/pull/633), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Run the provided rg command against docs\xml to list the 15 remaining placeholders under Javax.Xml.Datatype and ns-Javax.Xml.Datatype.xml. Read the surrounding XML documentation and the corresponding API entries, then replace each placeholder with accurate text; done means the scan returns no remaining matches in those paths.

Written by the indexing model from the issue text.