Report ashift of L2ARC devices in zdb

Commit 8af1104f does not actually store the ashift of cache devices in
their label. However, in order to facilitate reporting the ashift
through zdb, we enable this in the present commit. We also document
how the retrieval of the ashift is done.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Amanakis <gamanakis@gmail.com>
Closes #15331