update_from_3.3.md: platformsh_prestart_cacheclear.sh changes

and might not be the only one to have changed since last time its config was generated. It's the opportunity to review everything.

Author: adriendupuis

docs/update_and_migration/from_3.3/update_from_3.3.md

@@ -315,6 +315,20 @@ Run the following scripts:
     composer update "symfony/*"
     ```
 
+#### Ibexa Cloud
+
+Update Platform.sh configuration for PHP and Varnish.
+
+Generate new configuration with the following command:
+
+```bash
+composer ibexa:setup --platformsh
+```
+
+Review the changes applied to `.platform.app.yaml`, `.platform/` and `bin/platformsh_prestart_cacheclear.sh`,
+merge with your custom settings if needed, and commit them to Git.
+
+
 ### v3.3.14
 
 #### VCL configuration