iwlwifi: clarify NOCOPY/DUP documentation

Clarify the documentation to indicate that these
flags can only be used at the end, i.e. after them
a copy TFD (no flags set) is invalid.

Reported-by: Inbal Hacohen <inbal.hacohen@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>