Add rationale & example to prefer_for_elements_to_map_fromIterable (dart-archive/linter#3287)

* Add rationale & example to prefer_for_elements_to_map_fromIterable

* Add single quotes to for statements

To improve clarity of the messages.

Author: Merrit

lib/src/rules/prefer_for_elements_to_map_fromIterable.dart

@@ -8,10 +8,19 @@ import 'package:analyzer/dart/ast/visitor.dart';
 
 import '../analyzer.dart';
 
-const _desc = r'Prefer for elements when building maps from iterables.';
+const _desc = r"Prefer 'for' elements when building maps from iterables.";
 
 const _details = r'''
-When building maps from iterables, it is preferable to use for elements.
+When building maps from iterables, it is preferable to use 'for' elements.
+
+Using 'for' elements brings several benefits including:
+
+- Performance
+- Flexibility
+- Readability
+- Improved type inference
+- Improved interaction with null safety
+
 
 **BAD:**
 ```dart
@@ -30,6 +39,16 @@ return {
     '${demo.routeName}': demo.buildRoute,
 };
 ```
+
+**GOOD:**
+```dart
+// Map<int, Student> is not required, type is inferred automatically.
+final pizzaRecipients = {
+  ...studentLeaders,
+  for (var student in classG)
+    if (student.isPassing) student.id: student,
+};
+```
 ''';
 
 class PreferForElementsToMapFromIterable extends LintRule {