Add XML documentation for Android.Icu.Math

The `Android.Icu.Math` namespace contains **49** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **45** to **49** (increased by **4**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Run the provided rg command against docs\xml to locate the 49 `To be added` placeholders under Android.Icu.Math. Read the surrounding API entries and related documentation patterns, then replace each placeholder with appropriate XML documentation and rerun the scan to confirm none remain.

Written by the indexing model from the issue text.