docs: Improve clarity of sem-context documentation to better explain behaviour (#377)

## 📝 Description
I've improved the clarity of this part of the documentation, to point
out possible shortfalls users may have (expecting sem-context to work
between promotions).

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

Author: tbogosavljevic

docs/docs/reference/toolbox.md

@@ -307,7 +307,7 @@ retry --times 5 --sleep 10 bundle install
 
 ## sem-context {#sem-context}
 
-This tool can be used to share key-value data between Semaphore jobs. It enables communication between jobs and pipelines within the same workflow.
+This tool enables sharing key-value data between Semaphore jobs within the same workflow. It carries values from parent jobs, and updates made within a workflow are inherited by promoted jobs. However, please note that values are not shared across different promotions.
 
 :::note