command description is required for `population` commands

[JinIgarashi]

It can be better if description is provided how to use a command.

description can be written like below

https://github.com/UNDP-Data/geo-cb-surge/blob/1f625c35456b0ea74897cf84f8a219a6f75893ef/cbsurge/admin/__init__.py#L51-L73

[JinIgarashi]

download command description was added #133, but sync and aggregate command description still needs to be added.