Document what happens if options are defined more than once

Currently, we're allowing users to set some options like DataDirectory or User as often as they want. I've looked around, but we don't tell anyone what will happen in that case - currently, the option that is defined later will win, and command-line options win over torrc options.

Did I just not find this anywhere, or is that more than an implementation detail? Do we want to warn when some option is specified twice, and it can only have one value?

Question raised by roconnor on #tor, who wants to package Tor for NixOS.