*.md: bring mediaType out of reserved status

Signed-off-by: Vincent Batts <[email protected]>

Author: vbatts

image-index.md

@@ -17,8 +17,9 @@ For the media type(s) that this document is compatible with, see the [matrix][ma
 
 - **`mediaType`** *string*
 
-  This property is *reserved* for use, to [maintain compatibility][matrix].
-  When used, this field contains the media type of this document, which differs from the [descriptor](descriptor.md#properties) use of `mediaType`.
+  This property SHOULD be used and [remain compatible][matrix] with earlier versions of this specification and with other similar external formats.
+  When used, this field MUST contain the media type `application/vnd.oci.image.index.v1+json`.
+  This field usage differs from the [descriptor](descriptor.md#properties) use of `mediaType`.
 
 - **`manifests`** *array of objects*
 

manifest.md

@@ -22,8 +22,9 @@ Unlike the [image index](image-index.md), which contains information about a set
 
 - **`mediaType`** *string*
 
-  This property is *reserved* for use, to [maintain compatibility](media-types.md#compatibility-matrix).
-  When used, this field contains the media type of this document, which differs from the [descriptor](descriptor.md#properties) use of `mediaType`.
+  This property SHOULD be used and [remain compatible](media-types.md#compatibility-matrix) with earlier versions of this specification and with other similar external formats.
+  When used, this field MUST contain the media type `application/vnd.oci.image.manifest.v1+json`.
+  This field usage differs from the [descriptor](descriptor.md#properties) use of `mediaType`.
 
 - **`config`** *[descriptor](descriptor.md)*