Clarify MISSING_IDENTIFIER descriptions for access tokens

[LarryHu0217]

What type of PR is this?

• correction

What this PR does / why we need it:

Makes the MISSING_IDENTIFIER descriptions neutral to the access-token flow in both the API Design Guide and CAMARA_common.yaml.

The previous wording referred specifically to a three-legged access token, which could mislead developers using either a two-legged flow without an explicit identifier or a three-legged flow where the identifier cannot be derived from the token.

Which issue(s) this PR fixes:

Fixes #370

Does this PR introduce a breaking change?

• Yes
• No

Special notes for reviewers:

The deprecated CreateSubscriptionUnprocessableEntity422 block in CAMARA_event_common.yaml is intentionally unchanged because its inline documentation marks that content as frozen. Current event responses reference the subject-specific examples updated in CAMARA_common.yaml.

Validation:

• parsed CAMARA_common.yaml and asserted all three updated descriptions
• verified both API Design Guide table rows retain their structure and contain the neutral wording
• verified no stale issue wording remains in the two target files
• checked the scoped staged diff for whitespace errors while allowing the files' existing CRLF convention

Changelog input

 release-note
- Clarified MISSING_IDENTIFIER descriptions so they apply to both two-legged and three-legged access-token flows

Additional documentation

This section can be blank.

docs

[PedroDiez]

LGTM