Update documentation for SA1413 to include rationale

documentation/SA1413.md

@@ -19,11 +19,19 @@
 
 ## Cause
 
-The last statement in a multi-line C# initializer is missing a trailing comma.
+The last statement in a multi-line C# initializer or list is missing a trailing comma.
+
+### Rationale
+
+This rule is specifically designed to work well with the most widely used source control systems as an aid to long-term
+code review. By placing a comma on the last line of a multi-line sequence, developers who append an item to the list or
+reorder the list at some point in the future will not need to modify any more lines than absolutely necessary for the
+change. As a result, the size of the subsequent code review is minimized and focused, and tools like **git blame**
+continue to show the original author and commit for the item that was previously last in the list.
 
 ## Rule description
 
-A violation of this rule occurs when the last statement of a C# initializer is missing a trailing comma.
+A violation of this rule occurs when the last statement of a C# initializer or list is missing a trailing comma.
 
 For example, the following code would generate one instance of this violation:
 
@@ -47,6 +55,8 @@ var x = new Barnacle
 };
 ```
 
+This diagnostic is also reported for other forms of comma-separated list, such as enum members.
+
 ## How to fix violations
 
 To fix a violation of this rule, add a trailing comma to the last statement in the initializer.