filters: don't encode empty set. API docs

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)

docs/sources/reference/api/docker_remote_api_v1.12.md

@@ -712,6 +712,16 @@ Copy files or folders of container `id`
           }
         ]
 
+
+    Query Parameters:
+
+     
+
+    -   **all** – 1/True/true or 0/False/false, default false
+    -   **filters** – a json encoded value of the filters (a map[string][]string) to process on the images list.
+        
+
+
 ### Create an image
 
 `POST /images/create`