Following, please find a quote from Tor FAQ :
"Restricting your entry nodes may also help against attackers who want to run a few Tor nodes and easily enumerate all of the Tor user IP addresses. (Even though they can't learn what destinations the users are talking to, they still might be able to do bad things with just a list of users.) However, that feature won't really become useful until we move to a "directory guard" design as well."
What does "directory guard" design means in the above quote?
What does the last sentence in the above quote is implying?