improve footnote about distinguishing cause of failure

papers/icfp2021/queue-impl.tex

@@ -166,7 +166,8 @@ The function~\enqueue repeatedly calls \tryenqueue until it succeeds;
 the latter can fail either because the buffer is full or because of a competing enqueuer.%
 %
 \footnote{\label{fn:distinguishfail}%
-  We currently do not distinguish between these two causes, but this is feasible.
+  The code does not distinguish between these two causes, but this is feasible
+  with only one more test.
 }
 
 When calling \tryenqueue, we start by reading the current value~\head\ of the