Docs Update - CLI Descriptions

[sunilarjun]

The CLI pages have some missing descriptions that need to be updated across versions. For example, https://fleet.rancher.io/0.12/cli/fleet-cli/cleanup/fleet_cleanup has a missing item under "* fleet - ". Descriptions are to be added in the product documentation and can be applied here as well. Reference: rancher/fleet-product-docs#39