I've sent a patch to make the descriptions more useful:

https://issues.guix.info/issue/42262

> I follow you.  Some tools, like `atool`, make that responsibility clear in their documentation.  Unfortunately, GStreamer is not as clear.  That's an upstream issue.

The problem is that, unlike atool, we have a level of indirection in the
package that has optional dependencies.

See, Epiphany, Nyxt, etc. depend on on WebKitGTK explicitly, and it is
WebKitGTK that makes use of these optional packages.  So when the user
installs Epiphany or Nyxt, they won't see the WebKitGTK description
which mentions these dependencies.

A solution would be to mention the GStreamer plugins in the description
of all WebKitGTK browsers, but this seems rather tedious.

Cheers!

-- 
Pierre Neidhardt
https://ambrevar.xyz/