Debian/Ubuntu: Expand disk clearing steps

This covers more scenarios, including the need to stop md arrays and
wipe MD partitions.

Signed-off-by: Richard Laager <rlaager@wiktel.com>
Requested-by: Larry Wagner <larrywagner0@gmail.com>

docs/Getting Started/Debian/Debian Buster Root on ZFS.rst

@@ -167,15 +167,28 @@ Step 2: Disk Formatting
 
 #. If you are re-using a disk, clear it as necessary:
 
-   If the disk was previously used in an MD array, zero the superblock::
+   If the disk was previously used in an MD array::
 
      apt install --yes mdadm
+
+     # See if one or more MD arrays are active:
+     cat /proc/mdstat
+     # If so, stop them (replace ``md0`` as required):
+     mdadm --stop /dev/md0
+
+     # For an array using the whole disk:
      mdadm --zero-superblock --force $DISK
+     # For an array using a partition:
+     mdadm --zero-superblock --force ${DISK}-part2
 
    Clear the partition table::
 
      sgdisk --zap-all $DISK
 
+   If you get a message about the kernel still using the old partition table,
+   reboot and start over (except that you can skip this step).
+
+
 #. Partition your disk(s):
 
    Run this if you need legacy (BIOS) booting::