AIP-193: Clarify whether NOT_FOUND should be returned by the List method when there are no matching resources

[gkf]

I am guessing the intent is that when no resources match a List request, an empty result should be returned and not a NOT_FOUND error? So NOT_FOUND should only be returned when the parent resource does not exist? The wording is less clear after the reorganization in 88d2d62 since it now refers to "the requested resource or parent" instead of "the requested parent resource". The child resources returned by List could be considered to be "requested resources".