iwlwifi: mvm: document structures used by commands

Add documentation to a lot of command IDs that links to the
appropriate structure(s) used with those IDs. In one case,
actually add and use a new struct for that purpose.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>