ice: document 'shadow-ram' devlink region

78ad87da ("ice: devlink: add shadow-ram region to snapshot Shadow RAM")
added support for the 'shadow-ram' devlink region, but did not document it
in the ice devlink documentation. Fix this.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>