Clarify remove_bads doc

ir/ir/rm_bads.c

@@ -118,6 +118,8 @@ static void block_remove_bads(ir_node *block, void *env)
 }
 
 /* Remove Bad nodes from Phi and Block inputs.
+ *
+ * This does NOT remove unreachable code.
  *
  * Postcondition: No Bad nodes.
  */