Add XML documentation for Android.Net.Nsd

The `Android.Net.Nsd` namespace contains **274** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **267** to **274** (increased by **7**). Related documentation PRs: [#611](https://github.com/dotnet/android-api-docs/pull/611), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start in docs\xml by running the provided ripgrep command for Android.Net.Nsd and reviewing the matching namespace and ns-Android.Net.Nsd.xml files. Replace the 274 `To be added` placeholders with XML documentation, excluding docs\xml\index.xml; the scan should find no remaining placeholders in this namespace.

Written by the indexing model from the issue text.