The document revision you are trying to save.
OptionalconflictingDoc: CBLDocument<D>The conflicting revision already stored in the database,
or undefined if the document has been deleted.
'replace' to save the document, overwriting the conflicting revision; 'revert' to leave the saved document as it is without reporting an error; 'fail' to throw a ConflictError.
A callback that resolves conflicts that arise while saving or deleting a CBLDocument, if another revision has been saved since the document was loaded. The callback can examine body revisions and make changes to
docif desired, then return 'replace' to proceed; or by returning 'revert' it can leave the existing revision in place; or by returning 'fail' it can cause a ConflictError to be thrown.