In discussing #6047 we realized that gh doesn't do a good job of documenting auth scopes. This issue covers:

• Adding help topic gh help scopes
  • List out current scopes with definitions as per https://docs.github.com/en/developers/apps/building-oauth-apps/scopes-for-oauth-apps#available-scopes
  • Include disclaimer about always finding the up to date list at https://docs.github.com/en/developers/apps/building-oauth-apps/scopes-for-oauth-apps#available-scopes
• Mentioning help topic in usage for gh auth login
• Mentioning help topic in usage for gu auth refresh