Documentation: Add an explanation of NFSv4 client identifiers

To enable NFSv4 to work correctly, NFSv4 client identifiers have
to be globally unique and persistent over client reboots. We
believe that in many cases, a good default identifier can be
chosen and set when a client system is imaged.

Because there are many different ways a system can be imaged,
provide an explanation of how NFSv4 client identifiers and
principals can be set by install scripts and imaging tools.

Additional cases, such as NFSv4 clients running in containers, also
need unique and persistent identifiers. The Linux NFS community
sets forth this explanation to aid those who create and manage
container environments.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>