Implement return hooks for Lua functions (zero-cost if disabled).

doc/status.html

@@ -90,8 +90,8 @@ known incompatibilities with standard Lua:
 <ul>
 <li>
 The Lua <b>debug API</b> is missing a couple of features (return
-hooks) and shows slightly different behavior (no per-coroutine hooks,
-no tail call counting).
+hooks for non-Lua functions) and shows slightly different behavior
+(no per-coroutine hooks, no tail call counting).
 </li>
 <li>
 <b>Bytecode</b> currently cannot be loaded or dumped. Note that