Extend E0106, E0261

Added an example that points out hardly obvious mistake one could make when writing impl for a new type.

src/librustc/diagnostics.rs

@@ -362,6 +362,10 @@ struct Foo1 { x: &bool }
               // ^ expected lifetime parameter
 struct Foo2<'a> { x: &'a bool } // correct
 
+impl Foo2 { ... }
+  // ^ expected lifetime parameter
+impl<'a> Foo2<'a> { ... } // correct
+
 struct Bar1 { x: Foo2 }
               // ^^^^ expected lifetime parameter
 struct Bar2<'a> { x: Foo2<'a> } // correct
@@ -772,6 +776,24 @@ struct Foo<'a> {
     x: &'a str,
 }
 ```
+
+Implementations need their own lifetime declarations:
+    
+```
+// error, undeclared lifetime
+impl Foo<'a> {
+    ...
+}
+```
+
+Which are declared like this:
+
+```
+// correct
+impl<'a> Foo<'a> {
+    ...
+}
+```
 "##,
 
 E0262: r##"