Mention onionperf "a" instances using guards

https://metrics.torproject.org/onionperf-buildtimes.html says "We disable Entry Guards in the Tor client configuration used by OnionPerf, to ensure a new guard is selected for each circuit we measure." However, that's not true for the "a" instances which do use entry guards and rotate them every 12h. We should update the text on the website accordingly.