immuclient help should mention "<command> --help" syntax

[federico-razzoli]

What would you like to be added or enhanced

In immuclient, the help command shows a list of available commands.

I suggest to add a line to inform us that we can run any command with the --help argument (eg: exec --help).

Why is this needed

It's hard to learn it otherwise.

[mmeloni]

It's already present, isnt?

[zaza81]

@federico-razzoli can you confirm you see --help for commands as well? In case yes, please close the ticket.