improve AccountingStart manual entry

background: relay operator question: https://lists.torproject.org/pipermail/tor-relays/2017-September/013068.html

Teor eplained it very well here, this should go into the manual page. https://lists.torproject.org/pipermail/tor-relays/2015-May/006956.html

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information