32eddbf join: Use `xref` instead of a raw URL to docs page

Authored and Committed by jflory7 2 years ago
    join: Use `xref` instead of a raw URL to docs page
    
    This updates a link to the Fedora Join docs to use a `xref` cross-module
    link instead of hard-coding a URL.
    
    Signed-off-by: Justin W. Flory (he/him) <jwf@redhat.com>