Document that Vidalia relies on the format of the 'tor --version' output

When Vidalia runs Tor, it first starts by running "tor --version", and then looking for the first line that starts with the (case-insensitive) string "Tor version ". It assumes the text after that string, up until the next space character, is Tor's version.

Should we add that note to the version-spec.txt, to be aware that we should try to avoid changing the output format of tor --version? Or is there a better place to document it?

Are there other output formats that external programs rely on?

(Ticket motivated by #3049 (moved) and #3076 (moved).)