Add XML documentation for Android.OS.Storage

The `Android.OS.Storage` namespace contains **71** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **69** to **71** (increased by **2**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start with the provided rg command under docs\xml to locate the 71 `To be added` placeholders in Android.OS.Storage, excluding docs\xml\index.xml. Read the surrounding XML documentation entries and update the placeholders with accurate API descriptions; done means the namespace scan no longer reports those placeholders.

索引モデルが issue の本文から書いたものです。