Updated Storage backend compatibility (markdown)

Storage-backend-compatibility.md

@@ -43,4 +43,5 @@ Some storage backends do not support all the features of S3:
 * does not support deleting user metadata keys, only replacing them
 * does not support listing in-progress multipart uploads
 * does not support object-level access control
-* exposes multipart upload parts as regular keys
+* exposes multipart upload parts as regular keys
+* multi-delete does not delete SLO segments