Change documentation about RouterStatusEntryV3 bandwidth to specify that unit is kilobytes

In https://github.com/torproject/stem/blob/master/stem/descriptor/router_status_entry.py#L553 it says that it is unit-less, but i think it is not completely right. Tor code uses kilobytes: https://github.com/torproject/tor/blob/master/src/feature/dirauth/bwauth.c#L191, https://github.com/torproject/torspec/blob/master/dir-spec.txt#L2387, https://github.com/torproject/tor/blob/master/src/feature/dirauth/dirvote.c#L2303. sbws and Torflow also divide by 1000 after scaling (which changes orders of magnitude): https://github.com/torproject/sbws/blob/master/sbws/lib/v3bwfile.py#L175