[docs] fix "Discarding return values" GH #1004

According to zefram docs/pdds/pdd19_pir.pod talks about
  sub_label($I0, $I1, $I2)
as "... discards any return values".
Actually this is not correct.
The quoted code is equivalent to

() = sub_label($I0, $I1, $I2)
It doesn't save any return values, but if result count checking is
enabled then returning any non-zero number of results will cause an
exception. Not quite the discarding that the documentation suggests.

I changed the wording to "does not save any return values."