docs: use cache debugging notes (#86937)

Closes: #86896

Author: icyJoseph

docs/01-app/03-api-reference/01-directives/use-cache.mdx

@@ -518,6 +518,24 @@ export default function ClientComponent({ action }) {
 
 ## Troubleshooting
 
+### Debugging cache behavior
+
+#### Verbose logging
+
+Set `NEXT_PRIVATE_DEBUG_CACHE=1` for verbose cache logging:
+
+```bash
+NEXT_PRIVATE_DEBUG_CACHE=1 npm run dev
+# or for production
+NEXT_PRIVATE_DEBUG_CACHE=1 npm run start
+```
+
+> **Good to know:** This environment variable also logs ISR and other caching mechanisms. See [Verifying correct production behavior](/docs/app/guides/incremental-static-regeneration#verifying-correct-production-behavior) for more details.
+
+#### Console log replays
+
+In development, console logs from cached functions appear with a `Cache` prefix.
+
 ### Build Hangs (Cache Timeout)
 
 If your build hangs, you're accessing Promises that resolve to dynamic or runtime data, created outside a `use cache` boundary. The cached function waits for data that can't resolve during the build, causing a timeout after 50 seconds.