ice: add documentation for device-caps region

The recent change by commit 8d7aab35 ("ice: implement snapshot for
device capabilities") to implement the device-caps region for the ice
driver forgot to document it.

Add documentation to the ice devlink documentation file describing the
new region and add some sample output to the shell commands provided as
an example.

Fixes: 8d7aab35

 ("ice: implement snapshot for device capabilities")
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>