Clean up E0186 explanation

src/librustc_error_codes/error_codes/E0186.md

@@ -2,7 +2,7 @@ An associated function for a trait was defined to be a method (i.e., to take a
 `self` parameter), but an implementation of the trait declared the same function
 to be static.
 
-Here's an example of this error:
+Erroneous code example:
 
 ```compile_fail,E0186
 trait Foo {
@@ -17,3 +17,19 @@ impl Foo for Bar {
     fn foo() {}
 }
 ```
+
+When a type implements a trait's associated function, it has to use the same
+signature. So in this case, since `Foo::foo` takes `self` as argument and
+does not return anything, its implementation on `Bar` should be the same:
+
+```
+trait Foo {
+    fn foo(&self);
+}
+
+struct Bar;
+
+impl Foo for Bar {
+    fn foo(&self) {} // ok!
+}
+```