Method update

src/ExtMsg.actor.cpp:1360–1360

	explicit ExtReplaceUpdate(bson::BSONObj const& replaceWith) : replaceWith(replaceWith) {}

	Future<Void> update(Reference<IReadWriteContext> document) override { return replacementActor(this, document); }

	std::string describe() override { return "ReplaceWith(" + replaceWith.toString() + ")"; }