Add XML documentation for Android.Net.Http

The `Android.Net.Http` namespace contains **833** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

```powershell
rg -n -i --glob 'Android.Net.Http/**' --glob 'ns-Android.Net.Http.xml' 'to be added' docs\xml
```

## Update log

- 2026-09-10: Refreshed the placeholder scan from **827** to **833** (increased by **6**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Run the provided rg command against docs\xml to inspect the 833 placeholders under Android.Net.Http, excluding docs\xml\index.xml. Review related documentation PR #644 for project conventions and replace the namespace's placeholders with appropriate XML documentation until the scan no longer reports them.

由索引模型根据 Issue 内容生成。