Document when to use pluginFfi vs plugin

[dcharkes]

Adds documentation from the questions answered in:

• https://github.com/flutter/flutter/issues/108974

Presubmit checklist

• [x] This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• [x] This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• [x] This PR uses semantic line breaks of 80 characters or fewer.