Update doc and switch flag to default false

Signed-off-by: James Hewitt <james.hewitt@uk.ibm.com>

docs/configuration.md

@@ -1221,16 +1221,16 @@ features. Each subsection defines such a feature with configurable behavior.
 
 ## `validation`
 
-```none
+```yaml
 validation:
-  manifests:
-    urls:
-      allow:
-        - ^https?://([^/]+\.)*example\.com/
-      deny:
-        - ^https?://www\.example\.com/
+  disabled: false
 ```
 
+Use these settings to configure what validation the registry performs on content.
+
+Validation is performed when content is uploaded to the registry. Changing these
+settings will not validate content that has already been accepting into the registry.
+
 ### `disabled`
 
 The `disabled` flag disables the other options in the `validation`
@@ -1243,6 +1243,16 @@ Use the `manifests` subsection to configure validation of manifests. If
 
 #### `urls`
 
+```yaml
+validation:
+  manifests:
+    urls:
+      allow:
+        - ^https?://([^/]+\.)*example\.com/
+      deny:
+        - ^https?://www\.example\.com/
+```
+
 The `allow` and `deny` options are each a list of
 [regular expressions](https://pkg.go.dev/regexp/syntax) that restrict the URLs in
 pushed manifests.
@@ -1258,24 +1268,40 @@ one of the `allow` regular expressions **and** one of the following holds:
 
 #### `imageindex`
 
+validation:
+  manifests:
+    imageindex:
+      images:
+        disabled: false
+        platforms:
+        - os: linux
+          architecture: amd64
+
 ##### `images`
 
-Set `exist` to false to disable validation that images exist when an image
-index manifest is uploaded.
+By default, when an image index is uploaded to the registry, the registry will
+validate that the images referenced by the index exist in the registry before
+accepting the image index.
+
+Set `disabled` to `true` to disable all validation that images exist when an image
+index manifest is uploaded. This allows image lists to be uploaded to the registry
+without their associated images.
 
-Set `platforms` to configure the set of platforms to validate the existence of.
-If a platform is included in this list and in the images contained within an index,
-the registry will validate that it exists in the registry. The registry will not
-validate the existence of other platform images in the index.
+Set `platforms` to selectively validate the existence of platforms within image
+index manifests. If a platform is included in this list and in the images contained
+within an index, the registry will validate that the platform specific image exists
+in the registry before accepting the index. The registry will not validate the
+existence of other platform specific images in the index.
 
-This parameter does not validate that the platforms are included in the index,
-if an image index is missing one or more platform images from this array it may
-still be accepted by the registry.
+This parameter does not validate that the configured platforms are included in every
+index. If an image index is missing one or more platform specific images configured
+here, it may still be accepted by the registry.
 
 Each platform is a map with two keys, `os` and `architecture`, as defined in the
 [OCI Image Index specification](https://github.com/opencontainers/image-spec/blob/main/image-index.md#image-index-property-descriptions).
 
-If `platforms` is an empty array, all images in the index must exist.
+If `platforms` is an empty array (default), all images in the index must exist for
+the index to be accepted.
 
 ## Example: Development configuration