docs: clarify use of PinStatus.id

[lidel]

This PR does not change the spec, it simplifies docs related to /pins endpoint

• /pins/{cid-of-pin-object} can be confusing because we mean PinStatus.id, but a field named cid is present in PinStatus.Pin.cid
• /pins/{id} has a clear connection to PinStatus.id and reduces cognitive overhead

Preview: https://ipfs.github.io/pinning-services-api-spec/#specUrl=https://raw.githubusercontent.com/ipfs/pinning-services-api-spec/docs/clarify-object-id/ipfs-pinning-service.yaml

[lidel]

Merging in preparation for v0.0.5, will check copy in bulk before tagging the release.