Clean up E0309 explanation

src/librustc_error_codes/error_codes/E0309.md

@@ -1,9 +1,7 @@
-The type definition contains some field whose type
-requires an outlives annotation. Outlives annotations
-(e.g., `T: 'a`) are used to guarantee that all the data in T is valid
-for at least the lifetime `'a`. This scenario most commonly
-arises when the type contains an associated type reference
-like `<T as SomeTrait<'a>>::Output`, as shown in this example:
+A parameter type is missing an explicit lifetime bound and may not live long
+enough.
+
+Erroneous code example:
 
 ```compile_fail,E0309
 // This won't compile because the applicable impl of
@@ -25,9 +23,15 @@ where
 }
 ```
 
-Here, the where clause `T: 'a` that appears on the impl is not known to be
-satisfied on the struct. To make this example compile, you have to add
-a where-clause like `T: 'a` to the struct definition:
+The type definition contains some field whose type requires an outlives
+annotation. Outlives annotations (e.g., `T: 'a`) are used to guarantee that all
+the data in T is valid for at least the lifetime `'a`. This scenario most
+commonly arises when the type contains an associated type reference like
+`<T as SomeTrait<'a>>::Output`, as shown in the previous code.
+
+There, the where clause `T: 'a` that appears on the impl is not known to be
+satisfied on the struct. To make this example compile, you have to add a
+where-clause like `T: 'a` to the struct definition:
 
 ```
 struct Foo<'a, T>