reg-notes.def: Fix up description of REG_NOALIAS

The description of the REG_NOALIAS note in reg-notes.def isn't quite
right. It describes it as being attached to call insns, but it is
instead attached to a move insn receiving the return value from a call.

This can be seen by looking at the code in calls.cc:expand_call which
attaches the note:

  emit_move_insn (temp, valreg);

  /* The return value from a malloc-like function cannot alias
     anything else.  */
  last = get_last_insn ();
  add_reg_note (last, REG_NOALIAS, temp);

gcc/ChangeLog:

	* reg-notes.def (NOALIAS): Correct comment.