Config fixes

[hcsa73]

• Add --session-time-limit to config unset
• Change config unset behavior to set as the default, instead of setting to "" (so it reverts config set)
• Fix config list not showing bool values
• Change --async default to bool type (so its value in the config file is always bool -- before it was bool or string)
• Rewrite descriptions. Most notably,
  • Move the config logic with env vars from config set to config list
  • Move flag details from examples to the flag descriptions