Restore accidentally removed content
    
    In commit 5c2f228a50295abcc875e557715c30b0052304a4 (which was originally
    done in the wiki) I accidentally removed a section which forbade the use
    of arch-conditionalized Source: or Patch: tags when I was writing up
    https://pagure.io/packaging-committee/issue/751.
    
    This commit re-adds a section on the topic, which is a rewriting of the
    section that used to be there.  I've modified it for clarity and to
    address questions which arose in a discussion on the matter that took
    place in this thread:
    https://lists.fedoraproject.org/archives/list/packaging@lists.fedoraproject.org/thread/47YGILXHWNPZSO7OPHM2Z3ALXTWYU5PK/