Add XML documentation for Android.Text

The `Android.Text` namespace contains **1,374** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

## Update log

- 2026-09-10: Refreshed the placeholder scan from **1,891** to **1,374** (reduced by **517**). Related documentation PRs: [#380](https://github.com/dotnet/android-api-docs/pull/380), [#418](https://github.com/dotnet/android-api-docs/pull/418), [#644](https://github.com/dotnet/android-api-docs/pull/644), [#664](https://github.com/dotnet/android-api-docs/pull/664).

Run the provided rg scan from docs\xml to list the 1,374 `To be added` placeholders under Android.Text, excluding docs\xml\index.xml. Read the matching Android.Text XML files and replace the placeholders with appropriate documentation; done means the scan no longer reports those placeholders.

Vom Indexierungsmodell aus dem Issue-Text verfasst.