descriptor: Require RFC-6838-compliant mediaTypes

Restore the MUST which was removed from the 15f9f74 version of this
commit.  My personal preference would be to just say:

  Values MUST comply with [RFC 6838][rfc6838].

But Stephen wanted a direct reference to 4.2 as well [1].

[1]: opencontainers#448 (comment)

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

descriptor.md

@@ -19,7 +19,8 @@ The following fields contain the primary properties that constitute a Descriptor
 
 - **`mediaType`** *string*
 
-  This REQUIRED property contains the MIME type of the referenced content.
+  This REQUIRED property contains the media type of the referenced content.
+  Values MUST comply with [RFC 6838][rfc6838], including the [naming requirements in its section 4.2][rfc6838-s4.2].
 
   The OCI image specification defines [several of its own MIME types](media-types.md) for resources defined in the specification.
 
@@ -139,4 +140,6 @@ In the following example, the descriptor indicates that the referenced manifest
 ```
 
 [rfc3986]: https://tools.ietf.org/html/rfc3986
+[rfc6838]: https://tools.ietf.org/html/rfc6838
+[rfc6838-s4.2]: https://tools.ietf.org/html/rfc6838#section-4.2
 [rfc7230-s2.7]: https://tools.ietf.org/html/rfc7230#section-2.7