layer: revamp for readability
layer: revamp for readability

When adding verbiage for restriction on duplicate entries in the tar
archive, it was not immediately clear where this information would be
organized.

This revamp seeks to make the document on the whole more suitable for
implementors and be interpretable for compliance.

Signed-off-by: Vincent Batts <d01a4772f0cffd3f005a7d7f01c59cd104c7a5fe@hashbangbash.com>

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