Clean up E0207 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0207.md

@@ -1,3 +1,19 @@
+A type or lifetime parameter that is specified for `impl` is not constrained.
+
+Erroneous code example:
+
+```compile_fail,E0207
+struct Foo;
+
+impl<T: Default> Foo {
+    // error: the type parameter `T` is not constrained by the impl trait, self
+    // type, or predicates [E0207]
+    fn get(&self) -> T {
+        <T as Default>::default()
+    }
+}
+```
+
 Any type parameter or lifetime parameter of an `impl` must meet at least one of
 the following criteria:
 
@@ -10,19 +26,7 @@ the following criteria:
 ### Error example 1
 
 Suppose we have a struct `Foo` and we would like to define some methods for it.
-The following definition leads to a compiler error:
-
-```compile_fail,E0207
-struct Foo;
-
-impl<T: Default> Foo {
-// error: the type parameter `T` is not constrained by the impl trait, self
-// type, or predicates [E0207]
-    fn get(&self) -> T {
-        <T as Default>::default()
-    }
-}
-```
+The previous code example has a definition which leads to a compiler error:
 
 The problem is that the parameter `T` does not appear in the implementing type
 (`Foo`) of the impl. In this case, we can fix the error by moving the type