Working group updates

- "refers" -> "refers field"
- "referrers" -> "referrers list"
- lower case for defined words in the spec
- change "SHOULD" to "MUST" for allowing a manifest with an unknown refers
- change `reference` to `digest` in the referrers API
- add an error code for an invalid referrers API request
- add a "*note*" to stress the risk of data loss

Signed-off-by: Brandon Mitchell <[email protected]>