Update docs with CLI docstring info

[Iain-S]

The CLI main function used to have the following info, which I would argue is better off elsewhere in the documentation:

   What it does do:

    \b
      - Add subscriptions to the Billing System
      - Approve credits for a subscription.
      - Allocate approved credits to a subscription.
      - Check all the approvals and allocations made for a subscription.
      - Get a summary of all the approvals, allocations and costs for all subscriptions.

    What it doesn't do:

    \b
      - Create subscriptions on Azure.
      - Add users to a subscription.

[joseph-palmer]

Would we still want it displaying from rctab --h? If so it would need to be in the docstring.

Otherwise, what do you think would be better to see in the help option?