H1406: jellyfish have no head

I did some research, and technically, a jellyfish does indeed have no head
(and, technically, no brain, just a "nerve net").  The part on top is a
"bell" used for buoyancy, not a head.

doc/fixes34.4

@@ -367,6 +367,7 @@ cutting a long worm in half would trigger segfault/accvio crash if the hit
 	took parent down to 1 hit point or if long worms had become extinct
 blinded invisible hero can't see self as invisible via ';' or '/'
 a hangup save while picking up gold from shop floor could duplicate that gold
+jellyfish do not technically have a head
 
 
 Platform- and/or Interface-Specific Fixes

src/monst.c

@@ -2806,7 +2806,7 @@ struct permonst _mons2[] = {
 	A(ATTK(AT_STNG, AD_DRST, 3, 3), NO_ATTK,
 	  NO_ATTK, NO_ATTK, NO_ATTK, NO_ATTK),
 	SIZ(80, 20, MS_SILENT, MZ_SMALL), MR_POISON, MR_POISON,
-	M1_SWIM|M1_AMPHIBIOUS|M1_SLITHY|M1_NOLIMBS|M1_NOTAKE|M1_POIS,
+	M1_SWIM|M1_AMPHIBIOUS|M1_SLITHY|M1_NOLIMBS|M1_NOHEAD|M1_NOTAKE|M1_POIS,
 	M2_HOSTILE, 0, CLR_BLUE),
     MON("piranha", S_EEL,
 	LVL(5, 12, 4, 0, 0), (G_GENO|G_NOGEN|G_SGROUP),