Update double-free-modern-cpp.md (#1192)

From a discussion in #1122 where I realized that the heading is hard to
translate.

I added a bit of commentary as part of #1083.

src/ownership/double-free-modern-cpp.md

@@ -1,4 +1,4 @@
-# Extra Work in Modern C++ 
+# Defensive Copies in Modern C++ 
 
 Modern C++ solves this differently:
 
@@ -49,3 +49,23 @@ After copy-assignment:
 :                           :     `- - - - - - - - - - - -'
 `- - - - - - - - - - - - - -'
 ```
+
+<details>
+
+Key points:
+
+- C++ has made a slightly different choice than Rust. Because `=` copies data,
+  the string data has to be cloned. Otherwise we would get a double-free when
+  either string goes out of scope.
+
+- C++ also has [`std::move`], which is used to indicate when a value may be
+  moved from. If the example had been `s2 = std::move(s1)`, no heap allocation
+  would take place. After the move, `s1` would be in a valid but unspecified
+  state. Unlike Rust, the programmer is allowed to keep using `s1`.
+
+- Unlike Rust, `=` in C++ can run arbitrary code as determined by the type
+  which is being copied or moved.
+
+[`std::move`]: https://en.cppreference.com/w/cpp/utility/move
+ 
+</details>