Add explanatory text for error E0599

Add a text description of this error instead of just example error code

src/librustc_typeck/diagnostics.rs

@@ -3986,6 +3986,10 @@ details.
 "##,
 
 E0599: r##"
+This error occurs when a method is used on a type which doesn't implement it:
+
+Erroneous code example:
+
 ```compile_fail,E0599
 struct Mouth;