Update device removal documentation

Make a minor update to the 'zpool remove' man page to clarify both
raidz and draid pools do not support removal, and change sector to
ashift which is what we actually care about.

Update the big theory comment in vdev_removal.c to accurately reflect
which types of vdevs can be removed. Furthermore, I've added some
discussion for the casual reader to briefly explain the top-level
vdev removal restrictions. This has been a common area of confusion
and it's not intuitive where they come from without understanding
the implementation details.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Closes #17847