I couldn't find one. We ought to update the why is voting important? topic in the Help Center (h/t Bill the Lizard), but I think it's even more important to add a little guidance to the vote up privilege page, as this is the most-likely place for somebody new to voting to see it. (Arguably we should also mention it on the downvote page, but I don't think we need to. Downvotes come later, and sockpuppet upvoting is a bigger problem than revenge downvoting.)
I suggest adding the following to the end of "why is voting important" (after existing text explaining the importance of voting):
While we encourage everyone to upvote great posts, the motivation for doing so needs to be anchored in the merits of the post, not the person who wrote it. So don't skew the system by voting specifically in ways that benefit your friends, family, or colleagues; it's not fair to the community.
(This text is adapted from the mod-message template for targeted voting.)
On the "vote up" privilege page, add the following after the section "when should I vote up?":
When shouldn't I vote?
When your vote is not motivated by the content. Don't vote for your friends just because they're your friends, or out of sympathy if a post is getting downvotes. Vote based only on the content.