docs: Add note for cache in forked prs (#497)

## 📝 Description

renderedtext/project-tasks#2647

## ✅ Checklist
- [x] ~I have tested this change~
- [x] This change requires documentation update

Author: VeljkoMaksimovic

docs/docs/using-semaphore/cache.md

@@ -32,6 +32,12 @@ Users running Semaphore On-Premise or [self-hosted agents](./self-hosted) need t
 
 :::
 
+:::note
+
+For security reasons, the cache is not accessible from workflows started by Forked Pull Requests.
+
+:::
+
 ## Language support {#languages}
 
 The cache tool can recognize the structure for various popular languages and frameworks and automatically figure out what files to store and retrieve.