Update constant.pm to reflect list inlinement

Update constant.pm to reflect list inlinement

I also removed ‘some symbols may be redefined without generating a
warning’.  I know not to what it refers.  It has been there as long as
constant.pm has.  If a constant is clobbered by another constant with
the same value, there is no warning, as it is harmless.  That may be
to what it refers, but we don’t need a caveat for that.