man: say that SYSEXT_SCOPE=initrd also applies to exitrds

man: say that SYSEXT_SCOPE=initrd also applies to exitrds

We generally do _not_ want the same sysexts to be loaded in both initrd and
exitrd phases. The environment is completely different and it's unlikely that
the same code can be useful in both places. Nevertheless, it can be useful in
_some_ cases, for example when the sysexts contains debugging tools.

I think we don't need to differentiate between initrds and exitrds through
SYSEXT_SCOPE, because the two types are made available in completely different
locations and loaded through a different mechanism, with very little chance of
an initrd being loaded as an exitrd without an explicit admin action (or the
other way around). So let's not complicate our code or definitions by an
explicit "exitrd" sysext designator, but just clarify that "initrd" also
encompasses exitrds in this context.