CLI: Update all docs to use unitctl

[javorszky]

We're in the process of replacing unitc with unitctl. As part of that we'd like to replace all mention of unitc on the unit documentation with unitc with the appropriate documentation either adjusted if unitctl works differently, or removed if unitctl does not have feature parity with unitc yet.

[Jcahilltorre]

I can update the documentation, but I am not sure I can confirm the feature parity with unitc.

Who can help provide the raw content for this change?

cc @avahahn @javorszky