Add note to api conventions about kind: List (#5903)

* Add note to api conventions about kind: List

* Wording change

* Fixed small typo

Author: Chris Negus

contributors/devel/sig-architecture/api-conventions.md

@@ -128,6 +128,11 @@ lists should support filtering by fields (see
 
    Examples: `PodList`, `ServiceList`, `NodeList`.
 
+Note that`kubectl` and other tools sometimes output collections of resources
+as `kind: List`. Keep in mind that `kind: List` is not part of the Kubernetes API; it is
+exposing an implementation detail from client-side code in those tools, used to
+handle groups of mixed resources.
+
 3. **Simple** kinds are used for specific actions on objects and for
 non-persistent entities.