Docs: Clarify Resource Server startup expectations

[Hann244]

Refine the documentation to clarify Resource Server startup expectations now that
Spring Boot 2.6+ defers OIDC discovery by default.

• Clarify that, by default, the authorization server does not need to be available at startup.
• Add a concrete code example for users who require eager validation and want startup to fail when the authorization server is unavailable.
• Remove conflicting or outdated statements implying that startup will always fail if the authorization server is down.

Fixes gh-16708