Made it clear Nightmare achievements are only when playing Roguelike

git-svn-id: http://svn.net-core.org/repos/t-engine4@6346 51575b47-30f0-44d4-a5cc-537603b46e54

game/modules/tome/data/birth/descriptors.lua

@@ -186,6 +186,7 @@ newBirthDescriptor{
 		"Unfair game setting",
 		"All zone levels increased by 50% + 3",
 		"All creature talent levels increased by 50%",
+		"Player can earn Nightmare version of achievements if also playing in Roguelike permadeath mode.",
 	},
 	descriptor_choices =
 	{