ELF notes: rework the description of disabling
    
    I changed the definitions in package-notes to not do anything if
    %_package_note_file is undefined. The other macros won't work if it is
    not defined anyway, so this is quite natural. With this, undefining
    %_package_note_file becomes the most comprehensive mechanism to disable
    the feature. (Undefining %_package_note_flags still works so backwards
    compat is preserved.)
    
    The new mechanism will work once
    https://src.fedoraproject.org/rpms/package-notes/pull-request/1 is merged.
    But I don't think it is necessary to add a version requirement here, since
    we're only providing the docs here.