Clarify that 'order' is an optional parameter in Conduit API methods

Summary: Fixes T8603. For automatic 'order' parameters provided by infrastructure en route to T7715, clarify that they are optional (we will use the default builtin order for the underlying Query if an order is not provided).

Test Plan: Used web UI to see "optional" hint.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8603

Differential Revision: https://secure.phabricator.com/D13342