config: provide more complete explanation of ChainID
config: provide more complete explanation of ChainID

Signed-off-by: Stephen J Day <63efd86fa22882ae89b6027b22a36e4db1f4214d@docker.com>

mattspencer-arm/image-spec,jonboulle/image-spec,jonboulle/image-spec,opencontainers/image-spec,vbatts/oci-image-spec,mattspencer-arm/image-spec,vbatts/oci-image-spec,opencontainers/image-spec