Added documentation to variables in defaults.yml

Fixes #580 (closed).

@ProfessorManhattan, please see if we need to add the other variables available in https://gitlab.com/megabyte-labs/gas-station/-/blob/master/environments/dev/host_vars/ubuntu/users.yml to the defaults file. This is the complete list that was used to test the playbook on Ubuntu 22.04, so this is a good input file.

About the apps variable, I remember working on something related to this variable and the apps.yml but I am unable to find it now. Please update the description as appropriate.