The basic fact of the matter is, Wikipedia is a top-ten website. The number of websites that are large enough to cause any noticeable effect on server performance by live mirroring is probably in the hundreds. Google could literally (I once did some quick calculations) hotlink a Wikimedia image on their front page without much slowing down the image servers. The only reasons Wikimedia has to discourage unapproved live mirroring are 1) it can and does get money from commercially-operated sites for that privilege and 2) we don't, in principle, want people using Wikipedia content without proper GFDL compliance. #2 is a pretty weak reason to spend developer-hours on whack-a-mole, and in the case of #1, practically all of the mirrors would either just stop using Wikipedia content or make use of dumps instead, gaining nothing for the Foundation. So if someone wants to make a script that will find and block these things, okay, but it's not a very high priority.
Or at least that's my two cents, as a non-sysadmin. By all means update the docs, scaring people is good. ;) It's a wiki, feel free. Your text looks okay (although I'm not clear on whether it's actually illegal to hotlink content without permission).