Add XML documentation for Android.Text.Method

The `Android.Text.Method` namespace contains **488** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

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

Start by running the provided rg command against docs\xml and inspect the Android.Text.Method source XML files plus ns-Android.Text.Method.xml. Replace the 488 "To be added" placeholders with accurate XML documentation, then rerun the search to confirm none remain.

索引モデルが issue の本文から書いたものです。