Fix Optionable::Unknown when setting auto_discover

When setting `sessions.default.options.auto_discover` in `config/mongoid.yml`, I get: `Optionable::Unknown`: :auto_discover is an unknown option. Valid options are: :write, :read, :database, :max_retries, :pool_size, :retry_interval, :refresh_interval, :down_interval, :ssl, :timeout, :instrumenter.

This patch fixes that, along with a doco inconsistency.