In this paper, we study the impact of the human-readable documentation of Web ontologies on the ability of human users to agree on the membership of instances according to a given ontology. We first introduce a model of the problem and then present a user study, in which we measured the impact of documentation features in schema.org on the quality of annotations with n = 73 study participants. The paper concludes with a discussion of implications for ontology design in the context of the Semantic Web.