It isn’t I have scoured all documents for this information. The only thing I can find is that letter issued around Christmas time years ago (2011 I believe). While that letter did state it being in the POS I have never been able to find any language as strong as that language. The only thing I found is that
DVC is in charge of the decor and no alterations to the decor can occur without their approval.
So myself as a new owner didn’t know of the rules and only knew of them from the boards (not saying I decorate just that no published document is readily available as you hinted at).