Clean up E0263 explanation

src/librustc_error_codes/error_codes/E0263.md

@@ -1,7 +1,16 @@
-A lifetime name cannot be declared more than once in the same scope. For
-example:
+A lifetime was declared more than once in the same scope.
+
+Erroneous code example:
 
 ```compile_fail,E0263
-// error, lifetime name `'a` declared twice in the same scope
-fn foo<'a, 'b, 'a>(x: &'a str, y: &'b str) { }
+fn foo<'a, 'b, 'a>(x: &'a str, y: &'b str, z: &'a str) { // error!
+}
+```
+
+Two lifetimes cannot have the same name. To fix this example, change
+the second `'a` lifetime into something else (`'c` for example):
+
+```
+fn foo<'a, 'b, 'c>(x: &'a str, y: &'b str, z: &'c str) { // ok!
+}
 ```