docs(specs): state remote manifest size as the payload's own bytes

Why

The cache writer stores a remote manifest's size as the payload's own byte length, for an image manifest and a manifest list alike, while this spec said a progressive tree footprint complete for an image manifest, so a caller sizing remote image storage from the spec counted the manifest JSON and none of the layers. It surfaced on thread e37a6163 of feat(managementapi): serve container remote ima... (!1747 - merged) • Hayley Swimelar • 19.4, which corrects the OpenAPI contract to the writer's reading and leaves this sentence the last stale copy once it merges.

Related to #314