Document rationale for Dictionary specialized overloads

Author: stmontgomery

Sources/Testing/Test+Macro.swift

@@ -402,7 +402,9 @@ extension Test {
   ///
   /// This initializer overload is specialized for dictionary collections, to
   /// efficiently de-structure their elements (which are known to be 2-tuples)
-  /// when appropriate.
+  /// when appropriate. This overload is distinct from those for other
+  /// collections of 2-tuples because the `Element` tuple type for
+  /// `Dictionary` includes labels (`(key: Key, value: Value)`).
   ///
   /// - Warning: This function is used to implement the `@Test` macro. Do not
   ///   call it directly.

Sources/Testing/Test.Case.Generator.swift

@@ -228,7 +228,9 @@ extension Test.Case {
     ///
     /// This initializer overload is specialized for dictionary collections, to
     /// efficiently de-structure their elements (which are known to be 2-tuples)
-    /// when appropriate.
+    /// when appropriate. This overload is distinct from those for other
+    /// collections of 2-tuples because the `Element` tuple type for
+    /// `Dictionary` includes labels (`(key: Key, value: Value)`).
     init<Key, Value>(
       arguments dictionary: Dictionary<Key, Value>,
       parameters: [Test.ParameterInfo],