documenation for BandwidthRate etc should mention TCP/IP overhead not included

The Tor manual page should mention that BandwidthRate, BandwidthBurst, etc control only application layer byte rates and that TCP/IP overhead adds approximately 17% to the total, or stated a different way the payload data rate is about 15% of total traffic.

Is easy to overlook this important detail and it matters greatly when one is configuring maximums in order to stay within ISP bandwidth allocations. ISPs count total traffic and over-limit charges can be extortionist. Is unpleasant to exceed them by accident.

The overhead accounting also matters if one is setting BandwidthRate for an unmetered Internet connection with the idea of minimizing TCP/IP congestion in preference to Tor router flow control.

Generally ISPs do not charge for MAC layer bytes which are not carried on WAN links and so the above estimates omit it.