docs: Add note about fuser and mount namespaces

docs: Add note about fuser and mount namespaces

fuser won't match block devices that are mounted by
processes in a different mount namespace. This is
because the file descriptor will be from the process
namespace, not the "viewer" (e.g. fuser) namespace.

References:
 #39

Signed-off-by: Craig Small <csmall@dropbear.xyz>