umount: add note about --lazy

umount: add note about --lazy

Unfortunately, it's pretty common that users on production systems use
lazy umount to fix some FS issues. The usual result is unwanted system
reboot, because -l is not the right way how to fix unreachable NFS or
mess with local FS with submounts.

Note that after lazy umount /proc/self/mountinfo does not contain the
FS entry, but kernel still references the FS. It makes it very
difficult to debug.

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=1566674
Suggested-by:  Steve Dickson <steved@redhat.com>
Signed-off-by: Karel Zak <kzak@redhat.com>