Document when to use pluginFfi vs plugin (flutter#7428)

* Document when to use pluginFfi vs plugin

Adds documentation from the questions answered in:

* flutter/flutter#108974

* clarify sentence

Author: dcharkes

src/development/packages-and-plugins/developing-packages.md

@@ -614,6 +614,16 @@ a section in [Supporting the new Android plugins APIs][].
 If you want to develop a package that calls into native APIs using
 Dart's FFI, you need to develop an FFI plugin package.
 
+Both FFI plugin packages and (non-FFI) plugin packages support
+bundling native code, but FFI plugin packages do not support
+method channels and do include method channel registration code.
+If you want to implement a plugin that uses both method channels
+and FFI, use a (non-FFI) plugin. You can chose per platform to
+use an FFI or (non-FFI) plugin.
+
+FFI plugin packages were introduced in Flutter 3.0, if you're
+targeting older Flutter versions, you can use a (non-FFI) plugin.
+
 ### Step 1: Create the package
 
 To create a starter FFI plugin package,