makechrootpkg: Add a comment warning about a bug in "sudo -i"

The bug isn't currently triggered, but I accidentally did trigger when I
was trying to modify the command a bit.  I figure a "caution" sign would be
helpful to any future developers.

makechrootpkg.in

@@ -241,6 +241,10 @@ EOF
 # so no global variables
 _chrootbuild() {
 	. /etc/profile
+	# Beware, there are some stupid arbitrary rules on how you can
+	# use "$" in arguments to commands with "sudo -i".  ${foo} or
+	# ${1} is OK, but $foo or $1 isn't.
+	# https://bugzilla.sudo.ws/show_bug.cgi?id=765
 	sudo -iu builduser bash -c 'cd /startdir; makepkg "$@"' -bash "$@"
 }