'__close' methods can yield in the return of a C function

When, inside a coroutine, a C function with to-be-closed slots return,
the corresponding metamethods can yield. ('__close' metamethods called
through 'lua_closeslot' still cannot yield, as there is no continuation
to go when resuming.)

manual/manual.of

@@ -3102,6 +3102,9 @@ Close the to-be-closed slot at the given index and set its value to @nil.
 The index must be the last index previously marked to be closed
 @see{lua_toclose} that is still active (that is, not closed yet).
 
+A @Lid{__close} metamethod cannot yield
+when called through this function.
+
 (Exceptionally, this function was introduced in release 5.4.3.
 It is not present in previous 5.4 releases.)