document that dochugw() return value is ignored

src/mon.c

@@ -1050,7 +1050,7 @@ movemon_singlemon(struct monst *mtmp)
             && fightm(mtmp))
             return FALSE; /* mon might have died */
     }
-    dochugw(mtmp, TRUE); /* otherwise just move the monster */
+    (void) dochugw(mtmp, TRUE); /* otherwise just move the monster */
     return FALSE;
 }