config: Simplify the paragraph contrasting diffIDs with layer digests
config: Simplify the paragraph contrasting diffIDs with layer digests

Trevor and I couldn't agree on more detailed wording [1], and this
paragraph was never normative anyway.

[1]: https://github.com/opencontainers/image-spec/pull/388#discussion_r96765401

Signed-off-by: Stephen J Day <63efd86fa22882ae89b6027b22a36e4db1f4214d@docker.com>
Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

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