Add XML documentation for Java.Interop

The `Java.Interop` namespace originally contained **2,817** exact `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

PR #660 imported **20** entries backed by exact official Java 21 Javadoc and merged in `eb93ede4aa3190e7e9207431c9b178507272ffcc`.

## Remaining work

**2,797 placeholders remain:**

| Count | Category |
|---:|---|
| 2,327 | Managed/JNI support types with no exact Android/Java type registration or official managed API source |
| 380 | Array-rank JNI signatures with no declared Java API detail page |
| 87 | Exact Java type exists, but the member is managed lifecycle/wrapper/state behavior without an exact Java registration |
| 2 | Exact source member exists, but no official parameter channel matches the managed receiver/renamed argument |
| 1 | Managed namespace with no declared Android/Java API page |

The broad Oracle JNI specification import attempted during #660 was removed after review because native tables, parameters, return conventions, and calling protocols do not exactly describe the managed wrappers. The remaining work therefore needs Java.Interop-specific managed documentation rather than copied JNI/Javadoc prose, unless new exact metadata mappings become available.

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

See #660 for the importer changes, validation, and review history.

Start by running the documented rg search across docs\xml for `Java.Interop` and `To be added`, then read #660 for the importer, validation, and review history. Done means replacing remaining placeholders with Java.Interop-specific managed documentation or newly supported exact metadata mappings, while preserving the stated exclusions and validation expectations.

Written by the indexing model from the issue text.