docs/fast-export: explain --anonymize more completely

The original commit made mention of this option, but not why
one might want it or how they might use it. Let's try to be
a little more thorough, and also explain how to confirm that
the output really is anonymous.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>