I think to update a document can not only provide the changed key-value such as UPDATE table SET field=value. because the document is so flexible, the server don't know what you really want to do -- do you want to modify an exist value of key? or do you want to add some key-values? or do you want to delete some key-values? so the best way to update a document is to replace the whole document. though this need at least 3 steps:
get the whole document and rev info.
merge the update key-value you wanted into whole document.
put changed whole document and rev info to update the document.
Am I right?