add Flutter doc guidance to clarify $user in flutter_style_todos

lib/src/rules/flutter_style_todos.dart

@@ -15,6 +15,10 @@ const _details = r'''
 
 **DO** Use Flutter TODO format.
 
+From the [Flutter docs](https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo#comments):
+
+> TODOs should include the string TODO in all caps, followed by the GitHub username of the person with the best context about the problem referenced by the TODO in parenthesis. A TODO is not a commitment that the person referenced will fix the problem, it is intended to be the person with enough context to explain the problem. Thus, when you create a TODO, it is almost always your username that is given.
+
 **GOOD:**
 ```
 // TODO(username): message.