It's not very obvious what to document it since it applies to all
WebKitGTK.

It's part of the "general software knowledge".  For instance, if you
install an archiver like `atool', it's your reponsibility to install the
backends (unzip, p7zip, etc.) to support the various archive formats.
Where to document this?  I don't know, but it's unclear that Guix is the
place.

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