Mention $BAR_NAME in "config/events" docs environment variable section

[rowdyrotifer]

Although $BAR_NAME is documented in config/tricks#multiple-bars, it might be helpful to mention it along with these other environment variables in config/events. I missed that this environment variable existed because I went looking for it there. Nearly ended up hacking it with basename $CONFIG_DIR because I thought that was the only way to get the context easily.