man: note that zdb operates directly on pool storage

A frequent misunderstanding is that zdb accesses the pool through the
kernel or filesystem, leading to confusion particularly when it can't
access something that it seems like it should be able to.

I've seen this confusion recently when zdb couldn't access a pool because
the user didn't have permission to read directly from the block devices,
and when it couldn't show attributes of encrypted files even though the
dataset was unlocked.

The manpage already speaks to another symptom of this, namely that zdb
may "behave erratically" on an active pool; here I'm trying to make that
a little more explicit.

Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #14539