Fish completion for docker trust

[eiais]

I should be able to get this done relatively soon.

I also noticed while looking at this that some descriptions are out of date. I'll update those as well.

[eiais]

Bash commands are also missing key and signer. Is that to avoid issues with them going an extra level of sub coommands?

[eiais]

cc @dnephin

[eiais]

The image subcommand also needs to be added for fish. There is also an issue where subcommands that are named the same show flags that are valid for any

[vdemeester]

@eiais yeah I think the fish completion needs an updates on lot's of commands/subcommands