Describe all advanced agent settings

//cc @narqo @kaero 

This commit is not documented right now:
https://github.com/nodules/asker/commit/1720dd0cd03887174c8dc9d2ad6a57ca922e9b90

Though originally it was a compatibility issue, we still can pass, for example, `rejectUnauthorized` which is handy for backends with self-signed certificate.