Add XML documentation for Android.Media.Midi

The `Android.Media.Midi` namespace contains **188** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **176** to **188** (increased by **12**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Run the provided rg scan under docs\xml to enumerate the 188 `To be added` entries in the Android.Media.Midi files, including ns-Android.Media.Midi.xml. Read the surrounding API entries and Android.Media.Midi reference material before updating each placeholder; done means the namespace entries have meaningful XML documentation and the placeholder scan returns no remaining matches.

Written by the indexing model from the issue text.