Add comment to explain test

Author: lucic71

tests/unit/redundant_copy_in_conversion.cpp

@@ -5,5 +5,13 @@ int main() {
   m[0] = 1;
   std::map<int, int>::iterator end = m.end();
   std::map<int, int>::const_iterator const_it = m.find(0);
+  // Comparing const_iterator with iterator forces an implicit conversion of
+  // `end` to const_iterator. The AST shape differs between platforms:
+  //
+  //   Linux: const_it == ConvertingCtor(end)
+  //   macOS: const_it == ConvertingCtor(CopyCtor(end))
+  //
+  // The extra inner CopyCtor on macOS would emit a redundant .clone() in the
+  // generated Rust. cpp2rust suppresses it so the output matches Linux.
   return const_it == end ? 0 : 1;
 }