Documentation: quickstart-autokit: be more verbose on variables

[perezmeyer]

Currently https://github.com/balena-os/leviathan/blob/master/documentation/quickstart-autokit.md#provision-your-autokit-into-a-leviathan-worker-fleet in it's step 5 mentions that some variables need to be set, but do not mention exactly where they should be set.

Asking @vipulgupta2048 he told me this should go on the fleet. Links to documentation on how to do that would be awesome for newcomers too.