[SSL] Update certificate-statuses.mdx

Author: ngayerie

src/content/docs/ssl/reference/certificate-statuses.mdx

@@ -21,6 +21,12 @@ Once you issue a certificate, it should be in **Pending Validation**, but change
 
 If you deactivate a certificate, it will become a **Deactivating** and then an **Inactive** status.
 
+:::note
+You may encounter the **Pending Cleanup** status too. 
+Old certificats aren't deleted until the replacement has been successfully issued, to ensure a hostname does not break while replacing a certificate pack. 
+When a new certificate successfully issues and replaces an existing certificate pack on the edge, we can be sure that the hostname on the cert is now covered by a another cert so its safe to delete the previous. Until then the status is **Pending Cleanup**.
+:::
+
 ## Custom certificates
 
 If you are using a [custom certificate](/ssl/edge-certificates/custom-certificates/) and your [zone status](/dns/zone-setups/reference/domain-status/) is **Pending** or **Moved**, your certificate may have a status of **Holding Deployment**.