Add XML documentation for Javax.Crypto.Spec

The `Javax.Crypto.Spec` namespace contains **30** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **44** to **30** (reduced by **14**). Related documentation PRs: [#415](https://github.com/dotnet/android-api-docs/pull/415), [#444](https://github.com/dotnet/android-api-docs/pull/444), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the provided rg scan in docs\xml for the Javax.Crypto.Spec namespace and review the matching XML documentation files. Replace the 30 remaining `To be added` placeholders with documentation, excluding docs\xml\index.xml. Done means the target scan no longer reports those placeholders.

Scritto dal modello di indicizzazione a partire dal testo della issue.