Adjust seenres when observed attack succeeds

If a monster sees an elemental attack succeed from some other creature
or environmental danger, it will be willing to try those attacks again.

src/potion.c

@@ -884,6 +884,7 @@ peffect_sleeping(struct obj *otmp)
         You("yawn.");
     } else {
         You("suddenly fall asleep!");
+        monstunseesu(M_SEEN_SLEEP);
         fall_asleep(-rn1(10, 25 - 12 * bcsign(otmp)), TRUE);
     }
 }