container-build: fix help text for --isolated argument
    
    When a user submits an isolated build to OSBS, that setting does not
    affect OSBS's bump_release plugin at all. Remove the language about
    auto-incrementing the release value from the --help text.
    
    Replace the --help language about "additional tags" with "floating
    tags". This matches the language that the OSBS user documentation uses.
    Also, OSBS supports a deprecated (!) "additional-tags" file in dist-git,
    so it's confusing to refer to "additional tags" here.
    
    Signed-off-by: Ken Dreyer <kdreyer@redhat.com>