Modify rule S6619: Add Dart language (#4154)

rules/S6619/dart/rule.adoc

@@ -25,7 +25,6 @@ class A {
 
 === Code examples
 
-
 ==== Noncompliant code example
 
 [source,dart,diff-id=1,diff-type=noncompliant]
@@ -54,4 +53,27 @@ class A {
 
 == Resources
 
-* https://dart.dev/tools/linter-rules/avoid_null_checks_in_equality_operators[Dart Lint rule]
+=== Documentation
+
+* Dart Docs - https://dart.dev/tools/linter-rules/avoid_null_checks_in_equality_operators[Dart Linter rule - avoid_null_checks_in_equality_operators]
+* Dart Docs - https://dart.dev/null-safety/understanding-null-safety[Dart Null Safety - Understanding null safety]
+
+ifdef::env-github,rspecator-view[]
+
+'''
+== Implementation Specification
+(visible only on this page)
+
+=== Message
+
+Unnecessary null comparison in implementation of '=='.
+
+=== Highlighting
+
+The entire null comparison expression, e.g. `o == null` or `o != null`.
+
+'''
+== Comments And Links
+(visible only on this page)
+
+endif::env-github,rspecator-view[]