docs: clarify L1 backfill after L2 hit

[ming1523]

Summary

• document the L1 hit, L2 hit, and L2 miss paths in the multi-level cache flow
• backfill the current instance L1 when L2 returns a hit

Validation

• git diff --check
• Markdownlint and VuePress build were not run because dependencies are not installed in the checkout

This keeps the change scoped to docs/database/redis/cache-basics.md.