Add XML documentation for Android.Media.Audiofx

The `Android.Media.Audiofx` namespace contains **689** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **687** to **689** (increased by **2**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the provided rg command against docs\xml to enumerate the 689 `To be added` placeholders under Android.Media.Audiofx, excluding docs\xml\index.xml. Read the matching source XML entries and replace the placeholders with documentation; done means the namespace's placeholder scan is clear and the XML remains valid.

Written by the indexing model from the issue text.