Add XML documentation for Android.Health.Connect.DataTypes.Units

The `Android.Health.Connect.DataTypes.Units` namespace contains **189** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

```powershell
rg -n -i --glob 'Android.Health.Connect.DataTypes.Units/**' --glob 'ns-Android.Health.Connect.DataTypes.Units.xml' 'to be added' docs\xml
```

## Update log

- 2026-09-10: Refreshed the placeholder scan from **167** to **189** (increased by **22**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the provided rg command against docs\xml to review the 189 placeholders under Android.Health.Connect.DataTypes.Units, excluding docs\xml\index.xml. Read the surrounding source XML documentation to determine the appropriate descriptions, then verify that the namespace no longer contains any 'To be added' placeholders.

Written by the indexing model from the issue text.