Inconsistent permissions for accessing private snippets
### Problem
Snippets explore feature (https://gitlab.com/explore/snippets) allows admin and auditor role to see private snippets of other users.
However, the Snippet API list endpoint (`api/v4/snippets`) is restricted to show only personal or public snippets for admin and auditors. But it works for private snippets if we request them directly by id (`api/v4/snippets/25`).
### Proposal
* Match the behavior between the GitLab UI and the API functionality.
* Update the documentation
Related issues:
* https://gitlab.com/gitlab-org/gitlab/-/issues/212729+
* https://gitlab.com/gitlab-org/gitlab/-/issues/209005+
issue