The data parameter of wbeditentity contains a JSON structure that is used to overwrite or modify an entity. That structure is documented in docs/change-op-serialization.wiki and similar documents for other Wikibase extensions. Links to the canonical location of these documents should be included in the self-documentation of the wbeditentity API module.

Implementation idea: ChangeOpDeserializer could get a getDocumentationUrl() method. The EditEntity class would iterate over all known entity types, and collect the documentation URLs from the respective ChangeOpDeserializers, and include them in the help.