Add XML documentation for Java.Security.Spec

The `Java.Security.Spec` namespace contains **180** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **157** to **180** (increased by **23**). Related documentation PRs: [#447](https://github.com/dotnet/android-api-docs/pull/447), [#557](https://github.com/dotnet/android-api-docs/pull/557), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start with the provided rg command under docs\xml to enumerate the 180 `To be added` placeholders in Java.Security.Spec, excluding docs\xml\index.xml. Read the matching XML entries and related documentation PRs #447, #557, and #644 for context. Done means replacing the identified placeholders with appropriate XML documentation.

Written by the indexing model from the issue text.