Clarify how to update the secret value

[renatgalimov]

My intuition tells me I must set the secret value in the config file, but that's wrong.

In the help text, I clarify how to instantiate the secret value.

Checklist before merging

• [ ] If have added tests.
• [x] Is this a new feature? If yes, please write one phrase about this update.
• [ ] I've attached relevant screenshots (if relevant).

[korvyashka]

It is good to add in the documentation, though I don't think secret would be ever set in the config - also same applies to the setting - and we want to merge them as well - it all will be just setting command.

[rusko124]

@renatgalimov I don't think the PR is still relevant with the changes in dev-branch