The only documentation of the "serialization" format used in the data parameter of action=wbeditentity I could find is some examples provided in the auto-generated API help (see e.g. Examples at https://www.wikidata.org/w/api.php?action=help&modules=wbeditentity), and some examples on the help page on mediawiki.org (https://www.mediawiki.org/wiki/Wikibase/API#wbeditentity). These are, I believe, not enough.

We should consider what would be the best way of documenting this format. Should it be a separate document in docs directory of the Wikibase repository? Once the current format is described it would also be nice (and so much easier) to update examples in both places listed above.

Extensions providing custom entity types (MediaInfo, Lexeme etc) should be also documenting their specific ChangeOps.