Forward reference to api docs

Author: carols10cents

src/ch09-03-to-panic-or-not-to-panic.md

@@ -140,7 +140,8 @@ If code calling `Guess::new` passed in a value that was not between 1 and 100,
 that would be a violation of the contract that `Guess::new` is relying on. This
 function needs to signal to the calling code that it has a bug somewhere
 leading to the contract violation. The conditions in which `Guess::new` might
-panic should be discussed in its public-facing API documentation.
+panic should be discussed in its public-facing API documentation, which we will
+cover in Chapter XX.
 
 Important to note is the `value` field of the `Guess` struct is private, so
 code using this struct may not set that value directly. Callers *must* use the