Update documentation for SA1413 to include rationale

Closes #2416

Author: sharwell

documentation/SA1413.md

@@ -21,6 +21,14 @@
 
 The last statement in a multi-line C# initializer 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.