Clarify GRUB's lack of support for sha512, skein, edonr

Clarify GRUB's lack of support for sha512, skein, edonr

zfs.8 correctly said that GRUB did not support them, but
zpool-features.5 said that "Booting off pools...is supported."  Now,
zpool-features.5 discusses GRUB specifically and indicates its lack of
support for these features.  Also, I have clarified the wording in both
places to indicate that the pool feature cannot be used.  It's not a
filesystem dataset thing, but pool-wide.

I described this as "cannot be used".  I think technically the feature
can be enabled, just not active.  However, the effect is essentially the
same: you cannot enable those checksum algorithms on any dataset in the
pool, so you might as well not enable the feature (which is just
pointing a loaded gun at your foot).  In the past, an argument could be
made that having all the features enabled was useful for simplicity, as
long as you didn't activate the GRUB-incompatible features, but that's
getting less and less realistic over time.  A user can still do that,
but we should not encourage that.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Laager <rlaager@wiktel.com>
Closes #8626
Closes #8446