update break-continue.md (#2287)

Move the speak note about "loop" into the page which we actually talk
about "loop". Also update the wording to make it more accurate.

---------

Co-authored-by: Jason Lin <lxj@google.com>

src/control-flow-basics/break-continue.md

@@ -27,3 +27,12 @@ fn main() {
     }
 }
 ```
+
+<details>
+
+Note that `loop` is the only looping construct which can returns a non-trivial
+value. This is because it's guaranteed to only return at a `break` statement
+(unlike `while` and `for` loops, which can also return when the condition
+fails).
+
+</details>