I'll just state that it should be obvious to any user that targeting a specific user and up/downvote their posts without regard for their actual content is a misuse of the voting system. There is no need for any warnings here, common sense should be enough.
Of course there are corner cases, the most common is when a user acquires a fan. A new user encountering Jon Skeet for the first time might look through all the top answers from him and upvote them. This is not malicious behaviour, but it will be reverted by the script. But that is not a problem at all, the script takes care of it and no moderator has to be involved. The user also won't be punished, moderators judge each case individually and such benign serial voting is rather easy to detect.
The biggest argument against any warning is that this is likely to make the job of the moderators a lot harder. Such a warning would encourage and remind users to spread out their votes, and to avoid the automatic script. Vote fraud and serial voting is easier to deal with if it is rather blatant, so this would make it harder for the moderators. It still would not allow users to serial vote without being detected, the script is just the first line of defense and there are other ways this is detected and dealt with. But the script is the least amount of effort, and all other methods require significant work from either the moderators or the SE team, as they have full access to vote data.