The DNSSEC Section of the DNS forwarding service, don't have to autocomplete support.
If you start typing one of the possible completions, it justs display the whole list of commands.
tom@gw01# set service dns forwarding dnssec of Possible completions: off None process-no-validate None process None log-fail None validate None
About the description,
I'm not fully into the topic, but I think, it should be something like:
Possible completions: off No DNSSEC validation process-no-validate ??? process ??? log-fail Log invalid DNSSEC Domains? validate Validate DNSSEC Records