That doesn't quite mesh with my understanding. Some media types support hyperlinks, like HTML, which describe how links should be included (anchors, forms and link elements).
For media types that don't support hyperlinks, such as images, video etc; you can use the HTTP header Link (http://www.w3.org/wiki/LinkHeader).
Both these methods support the relationship attribute allowing you to specify the relationship between resources thus documenting the way clients should interact with such linked hypermedia (e.g. the HTML media type specifies that Link elements with the rel attribute should perform a HTTP GET request to the indicated resource).
In the example you gave, you could use a rel attribute like "comment" and link to the comments. RESTful clients could then follow the hyperlinks to the comments, knowing how to interact with them and what to do:
<link rel="comment" href="/comments/2398293"/>
I don't think this is, in practice, a huge deal, because people don't write generic consumers of RESTful APIs. At some point you need to actually know what it is you're trying to do and code for it.
On top of that, with RDF or XLink or XPointer and whatnot there probably are standards for doing this kind of thing, but nobody knows them, uses them or follows them.