Add XML documentation for Java.Security.Cert

The `Java.Security.Cert` namespace contains **240** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **291** to **240** (reduced by **51**). Related documentation PRs: [#406](https://github.com/dotnet/android-api-docs/pull/406), [#464](https://github.com/dotnet/android-api-docs/pull/464), [#497](https://github.com/dotnet/android-api-docs/pull/497), [#508](https://github.com/dotnet/android-api-docs/pull/508), [#537](https://github.com/dotnet/android-api-docs/pull/537), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the provided rg scan against docs\xml, then inspect the Java.Security.Cert/** files and ns-Java.Security.Cert.xml entries it reports. Replace the 240 `To be added` placeholders with appropriate XML documentation and rerun the scan to confirm the targeted placeholders have been addressed.

Written by the indexing model from the issue text.