Koji doc: discourage use of direct chained builds
    
    Direct chained builds really aren't a good idea any more, now we
    create updates for all Rawhide builds and apply gating tests to
    some. If a build in the chain fails gating tests it will not be
    pushed stable and the chain will fail.
    
    This explains that, but also explains that you can use
    `chain-build --target` to do a chain build to a side tag, and
    this might be a nice workflow.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>