Update generator and SDK doc with current confg options

[waltjones]

References: https://github.com/rollbar/rollbar-gem/pull/867#issuecomment-604858333

Update the initializer template with current config options: https://github.com/rollbar/rollbar-gem/blob/master/lib/generators/rollbar/templates/initializer.rb

And update the config reference with the options in the referenced comment. https://docs.rollbar.com/docs/gem-configuration-reference