Document in control-spec.txt that GETINFO dir/server/* won't work with microdescriptors turned on

12:29:00 < gamambel> a user is asking why tor returns 551 server 
                     unavailable for "getinfo dir/server/all" with 
                     0.2.3.x. it used to work in 0.2.2.x, and as far 
                     as i can see it is still in the control-spec?
13:33:21 < karsten> gamambel: ah! tor doesn't download server 
                    descriptors anymore, but microdescriptors.
13:33:34 < karsten> gamambel: so, tor is right. it doesn't have those 
                    server descriptors.
13:33:49 < karsten> maaaaybe that should be documented in 
                    control-spec.txt
13:34:06 < karsten> as a helpful comment

Such a comment could be: "Note that, as of Tor 0.2.3.3-alpha, Tor doesn't download server descriptors anymore, but microdescriptors. So, a "551 Servers unavailable" reply to all "GETINFO dir/server/*" requests is actually correct."