Clean up E0373 explanation

src/librustc_error_codes/error_codes/E0373.md

@@ -1,6 +1,6 @@
-This error occurs when an attempt is made to use data captured by a closure,
-when that data may no longer exist. It's most commonly seen when attempting to
-return a closure:
+A captured variable in a closure may not live long enough.
+
+Erroneous code example:
 
 ```compile_fail,E0373
 fn foo() -> Box<Fn(u32) -> u32> {
@@ -9,6 +9,10 @@ fn foo() -> Box<Fn(u32) -> u32> {
 }
 ```
 
+This error occurs when an attempt is made to use data captured by a closure,
+when that data may no longer exist. It's most commonly seen when attempting to
+return a closure as shown in the previous code example.
+
 Notice that `x` is stack-allocated by `foo()`. By default, Rust captures
 closed-over data by reference. This means that once `foo()` returns, `x` no
 longer exists. An attempt to access `x` within the closure would thus be