docs: emails in cloud (#14040)

* docs: emails in cloud

* add notes to user guides

* small updates

* small change

* fix link

* addressed feedback

www/apps/cloud/app/environments/environment-variables/page.mdx

@@ -38,6 +38,9 @@ The following environment variable names are reserved by Cloud and you can't use
 - `AWS_ACCESS_KEY_ID`
 - `AWS_SECRET_ACCESS_KEY`
 - `AWS_REGION`
+- `MEDUSA_CLOUD_API_KEY`
+- `MEDUSA_CLOUD_ENVIRONMENT_HANDLE`
+- `MEDUSA_CLOUD_EMAILS_ENDPOINT`
 
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