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).