Restore the sentance that constraints break and continue.

This requires break and continue to target blocks in constructs they are
nested in.

Author: sunfishcode

AstSemantics.md

@@ -253,9 +253,10 @@ are statements.
   * `switch`: switch statement with fallthrough
 
 Loops (`do_while` and `forever`) may only be entered via fallthrough at the top.
-In particular, they may not be entered directly via a `break`, `continue`, or
-`switch` destination. This guarantees that all control flow graphs are
-well-structured.
+In particular, loops may not be entered directly via a `break`, `continue`, or
+`switch` destination. Break and continue statements can only target blocks or
+loops in which they are nested. These rules guarantee that all control flow
+graphs are well-structured.
 
 Structured control flow provides simple and size-efficient binary encoding and
 compilation. Any control flow—even irreducible—can be transformed into structured