Add XML documentation for Android.OS.Strictmode

The `Android.OS.Strictmode` namespace contains **258** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

Use the provided rg command under docs\xml to locate the 258 `To be added` placeholders in Android.OS.Strictmode source XML documentation, excluding docs\xml\index.xml. Read the surrounding entries to determine the needed API descriptions, then verify that no matching placeholders remain in the namespace files.

索引モデルが issue の本文から書いたものです。