Add XML documentation for System.Drawing

The `System.Drawing` namespace contains **1,246** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **1,316** to **1,246** (reduced by **70**). Related documentation PRs: [#591](https://github.com/dotnet/android-api-docs/pull/591), [#644](https://github.com/dotnet/android-api-docs/pull/644).

Run the provided rg scan in docs\xml to enumerate the remaining `To be added` placeholders under System.Drawing, excluding docs\xml\index.xml. Start with the matching System.Drawing/** files and ns-System.Drawing.xml, then replace the placeholders with XML documentation. Done means the scan no longer reports those placeholders; review related PRs #591 and #644 for context.

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