tutorial: add an example of freezing a managed box

doc/tutorial.md

@@ -1173,10 +1173,7 @@ For a more in-depth explanation of borrowed pointers, read the
 ## Freezing
 
 Borrowing an immutable pointer to an object freezes it and prevents mutation.
-`Owned` objects have freezing enforced statically at compile-time. Mutable
+`Owned` objects have freezing enforced statically at compile-time.
-managed boxes handle freezing dynamically when any of their contents are
-borrowed, and the task will fail if an attempt to modify them is made while
-they are frozen.
 
 ~~~~
 let mut x = 5;
@@ -1186,6 +1183,20 @@ let mut x = 5;
 // x is now unfrozen again
 ~~~~
 
+Mutable managed boxes handle freezing dynamically when any of their contents
+are borrowed, and the task will fail if an attempt to modify them is made while
+they are frozen:
+
+~~~~
+let x = @mut 5;
+let y = x;
+{
+    let y = &*y; // the managed box is now frozen
+    // modifying it through x or y will cause a task failure
+}
+// the box is now unfrozen again
+~~~~
+
 # Dereferencing pointers
 
 Rust uses the unary star operator (`*`) to access the contents of a