Monsters can see player resistances

If monsters see you resist something, generally elemental or magical
attack, or if they see you reflect an attack, they learn that and
will adjust their attack accordingly.

Originally from SporkHack, but this version comes via EvilHack with
some minor changes.

src/potion.c

@@ -796,6 +796,7 @@ peffects(struct obj *otmp)
         break;
     case POT_SLEEPING:
         if (Sleep_resistance || Free_action) {
+            monstseesu(M_SEEN_SLEEP);
             You("yawn.");
         } else {
             You("suddenly fall asleep!");
@@ -1729,8 +1730,10 @@ potionbreathe(struct obj *obj)
             g.multi_reason = "sleeping off a magical draught";
             g.nomovemsg = You_can_move_again;
             exercise(A_DEX, FALSE);
-        } else
+        } else {
             You("yawn.");
+            monstseesu(M_SEEN_SLEEP);
+        }
         break;
     case POT_SPEED:
         if (!Fast)