Add XML documentation for Android.Content.OM

The `Android.Content.OM` namespace contains **97** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **91** to **97** (increased by **6**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start in docs\xml and run the issue's ripgrep command to enumerate the 97 "To be added" placeholders under Android.Content.OM. Review the surrounding XML documentation for each entry and replace the placeholders with accurate descriptions; done means the namespace scan reports no remaining placeholders, excluding docs\xml\index.xml.

索引モデルが issue の本文から書いたものです。