Add XML documentation for Javax.Crypto

The `Javax.Crypto` namespace contains **44** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **47** to **44** (reduced by **3**). Related documentation PRs: [#383](https://github.com/dotnet/android-api-docs/pull/383), [#502](https://github.com/dotnet/android-api-docs/pull/502), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Run the provided rg command from the repository root to list the 44 remaining placeholders under docs\xml for Javax.Crypto, excluding docs\xml\index.xml. Read the surrounding XML entries to identify each API's intended documentation, then replace every "To be added" placeholder with appropriate text. Done means the scan returns no remaining placeholders in the targeted files.

索引モデルが issue の本文から書いたものです。