summarize the modifications

content/en/docs/tasks/access-kubernetes-api/custom-resources/custom-resource-definitions.md

@@ -366,11 +366,42 @@ You can enable the feature using the `CustomResourcePublishOpenAPI` feature gate
 
 Custom resource validation schema will be converted to OpenAPI v2 schema, and
 show up in `definitions` and `paths` fields in the OpenAPI spec.
-Additional modification is applied during the conversion to keep backwards compatiblity with
-kubectl in previous 1.13 version. The conversion won't modify the validation schema defined in CRD,
-and therefore won't affect [validation](#validation) in API server. See
-[conversion implementation](https://github.com/kubernetes/kubernetes/blob/83ff0f6c64485ffe491d5116596072f466be1bd5/staging/src/k8s.io/apiextensions-apiserver/pkg/controller/openapi/conversion.go#L42-L112)
-and [compatibility analysis](https://github.com/kubernetes/kube-openapi/pull/143) for detail.
+The following additional modifications are applied during the conversion to keep backwards compatiblity with
+kubectl in previous 1.13 version. These modifications prevent kubectl from being over-strict and rejecting
+valid openapi schemas that it doesn't understand. The conversion won't modify the validation schema defined in CRD,
+and therefore won't affect [validation](#validation) in API server.
+
+1. The following fields are removed as they aren't supported by OpenAPI v2
+
+- The fields `oneOf`, `anyOf` and `not` are removed
+
+2. The following fields are removed as they aren't allowed by kubectl in
+   previous 1.13 version
+
+- For a schema with a `$ref`
+  - the fields `properties` and `type` are removed
+  - if the `$ref` is outside of the `definitions`, the field `$ref` is removed
+- For a schema of a primitive data type (which means the field `type` has two elements: one type and one format)
+  - if any one of the two elements is `null`, the field `type` is removed
+  - otherwise, the fields `type` and `properties` are removed
+- For a schema of more than two types
+  - the fields `type` and `properties` are removed
+- For a schema of `null` type
+  - the field `type` is removed
+- For a schema of `array` type
+  - if the schema doesn't have exact one item, the fields `type` and `items` are
+    removed
+- For a schema with no type specified
+  - the field `properties` is removed
+
+3. The following fields are removed as they aren't supported by gnostic
+
+- The fields `id`, `schema`, `definitions`, `additionalItems`, `dependencies`,
+  and `patternProperties` are removed
+- For a schema with a `externalDocs`
+  - if the `externalDocs` has `url` defined, the field `externalDocs` is removed
+- For a schema with `items` defined
+  - if the field `items` has multiple schemas, the field `items` is removed
 
 ### Additional printer columns