Add clarification of default error codes

Author: Konstiantyn Gominyuk

Tests/Foundation/Tests/TestNSError.swift

@@ -149,12 +149,13 @@ class TestNSError : XCTestCase {
     }
 
     func test_ConvertErrorToNSError_errorCodeWithAssosiatedValue() {
+        // Default error code for enum case is based on EnumImplStrategy::getTagIndex
         enum SwiftError: Error {
-            case one
-            case two
-            case three(String)
-            case four
-            case five(String)
+            case one // 2
+            case two // 3
+            case three(String) // 0
+            case four // 4
+            case five(String) // 1
         }
 
         XCTAssertEqual((SwiftError.one as NSError).code, 2)