Explain gotoifnot a bit more carefully

Author: willtebbutt

src/copyable_task.jl

@@ -255,7 +255,12 @@ The second component of the transformation is implementing the `produce` mechani
 ability to resume computation from where we produced. Roughly speaking, the `IRCode` must be
 modified to ensure that whenever a `produce` call in encountered, the `MistyClosure` returns
 the argument to `produce`, and that subsequent calls resume computation immediately after
-the `produce` statement. Observe that this is also facilitated by the ref mechanism
+the `produce` statement. This resumption is achieved by setting the value of a counter
+prior to returning following a `produce` statement -- a sequence of comparisons against this
+counter, and `goto-if-not` statement are inserted at the top of the IR. These are used to
+jump to the point in the code from which computation should resume. These are set up such
+that, when the `TapedTask` is first run, computation start froms the first statement.
+Observe that this is also facilitated by the ref mechanism
 discussed above, as it ensures that the state persists between calls to a `MistyClosure`.
 
 The above gives the broad outline of how `TapedTask`s are implemented. We refer interested