Clean up E0310 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0310.md

@@ -1,7 +1,7 @@
-Types in type definitions have lifetimes associated with them that represent
-how long the data stored within them is guaranteed to be live. This lifetime
-must be as long as the data needs to be alive, and missing the constraint that
-denotes this will cause this error.
+A parameter type is missing a lifetime constraint or has a lifetime that
+does not live long enough.
+
+Erroneous code example:
 
 ```compile_fail,E0310
 // This won't compile because T is not constrained to the static lifetime
@@ -11,6 +11,11 @@ struct Foo<T> {
 }
 ```
 
+Type parameters in type definitions have lifetimes associated with them that
+represent how long the data stored within them is guaranteed to live. This
+lifetime must be as long as the data needs to be alive, and missing the
+constraint that denotes this will cause this error.
+
 This will compile, because it has the constraint on the type parameter:
 
 ```