Add XML documentation for Java.Security

The `Java.Security` namespace contains **460** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **476** to **460** (reduced by **16**). Related documentation PRs: [#384](https://github.com/dotnet/android-api-docs/pull/384), [#390](https://github.com/dotnet/android-api-docs/pull/390), [#477](https://github.com/dotnet/android-api-docs/pull/477), [#505](https://github.com/dotnet/android-api-docs/pull/505), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Run the provided rg scan under docs\xml, limited to Java.Security/** and ns-Java.Security.xml, to review the 460 remaining placeholders. Read the surrounding source XML documentation and related Java.Security API context before documenting entries. Done means the targeted placeholders have been replaced with useful documentation and the scan no longer reports them.

Written by the indexing model from the issue text.