Clarify API coverage of IP restriction

What does this MR do?

The IP restriction is enforced by disabling permissions to read a project or group resource to the authenticated user when disallowed due to their request IP.

This effectively disables all possibility of reading the group or project metadata, and therefore covers all APIs of form /groups/:id[/*] or /projects/:id[/*]. In a test everything under these resources began to yield 404s when requested from a disallowed IP.

The current document entry links to specific API resources that imply only certain endpoints are restricted.

This change attempts to clarify that every project and group related API resource is covered by linking instead to the API resources documentation page.

Related issues

Customer ticket: https://gitlab.zendesk.com/agent/tickets/256505

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