lots of documentation

booksrc/tutorial11.md

@@ -1,6 +1,23 @@
 # Debug for the ErrorKind
 
-[TBD]
+One small improvement at the end of the tutorial is to fix the debug output of
+`Func1ErrorKind`. As you probably noticed, the output doesn't say much of the enum.
+
+~~~
+Debug Error:
+src/main.rs:35: Func2
+[…]
+~~~
+
+As a lazy shortcut, we implement `Debug` by calling `Display` and end up with
+
+~~~
+Debug Error:
+src/main.rs:40: func1 error calling func2
+[…}
+~~~
+
+which gives us a lot more detail.
 
 ~~~rust
 use crate::chainerror::*;