retro_waitable_spsc: say what the write notification actually does

The comment on the producer's notify opened by claiming the consumer is
told even when nothing was written, then spent the rest of the sentence
explaining that the notify is skipped. The code skips it. What makes
that safe is that a write fitting nothing leaves the queue exactly as
the consumer last saw it, and a consumer parks only on a queue with
nothing to read, which a queue too full to accept the write is not.

libretro-common/queues/retro_waitable_spsc.c

@@ -90,9 +90,11 @@ size_t retro_waitable_spsc_write(retro_waitable_spsc_t *q,
 {
    size_t n = retro_spsc_write(&q->queue, data, bytes);
 
-   /* Told even when nothing was written: a consumer parked on a queue
-    * this full is waiting on the producer, and a write that fitted
-    * nothing has not changed that, so the notify is skipped. */
+   /* Skipped when nothing was written: a write that fitted nothing
+    * leaves the queue exactly as the consumer last saw it, and a
+    * consumer only parks on a queue with nothing to read - which a
+    * queue too full to accept this write is not. There is no wake to
+    * lose. */
    if (n)
       retro_eventcount_notify(&q->readable);