Clean up E0072 long explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0072.md

@@ -1,20 +1,23 @@
-When defining a recursive struct or enum, any use of the type being defined
-from inside the definition must occur behind a pointer (like `Box` or `&`).
-This is because structs and enums must have a well-defined size, and without
-the pointer, the size of the type would need to be unbounded.
+A recursive type has infinite size because it doesn't have an indirection.
 
-Consider the following erroneous definition of a type for a list of bytes:
+Erroneous code example:
 
 ```compile_fail,E0072
-// error, invalid recursive struct type
 struct ListNode {
     head: u8,
-    tail: Option<ListNode>,
+    tail: Option<ListNode>, // error: no indirection here so impossible to
+                            //        compute the type's size
 }
 ```
 
-This type cannot have a well-defined size, because it needs to be arbitrarily
-large (since we would be able to nest `ListNode`s to any depth). Specifically,
+When defining a recursive struct or enum, any use of the type being defined
+from inside the definition must occur behind a pointer (like `Box`, `&` or
+`Rc`). This is because structs and enums must have a well-defined size, and
+without the pointer, the size of the type would need to be unbounded.
+
+In the example, the type cannot have a well-defined size, because it needs to be
+arbitrarily large (since we would be able to nest `ListNode`s to any depth).
+Specifically,
 
 ```plain
 size of `ListNode` = 1 byte for `head`