docs(nuxt-better-auth): clarify/standardize NuxtHub KV secondaryStorage session behavior

[onmax]

Motivation

NuxtHub KV secondaryStorage usage is currently under-specified for operational rollouts.

Scope

• Define authoritative baseline strategy (DB-default first).
• Add explicit checklist for enabling KV secondary session storage.
• Document SSR/invalidation consistency caveats.
• Provide migration rollout and rollback playbooks.

Acceptance Criteria

• Clear default recommendation and decision rubric.
• Concrete examples for DB-default and KV-secondary configs.
• Reduced ambiguity for production rollout decisions.

Implementation

PR: #142