Add diverging loops to diverging function section.

I went with "corresponding break" here since you could have a
break that's inside another loop expression for the inner loop
expression, but that wouldn't change the divergence. That probably
needs to be better fleshed out, but I'd leave the fleshing out
of divergence to the loop item section of the reference.

Author: Havvy

src/items.md

@@ -365,7 +365,8 @@ fn my_err(s: &str) -> ! {
 ```
 
 We call such functions "diverging" because they never return a value to the
-caller. Every control path in a diverging function must end with a `panic!()` or
+caller. Every control path in a diverging function must end with a `panic!()`,
+a loop expression that does not contain a corresponding break expression, or
 a call to another diverging function on every control path. The `!` annotation
 does *not* denote a type.