Update `manual.conf' configuration file used to produced documetation
    manuals written in Texinfo format.
    
        - Add `manual_backend' option to list of configurable options.
          Certainly, this option must not be changed. It is there to save
          you from typing `--backend=texinfo' option everytime in the
          command line when more than one documentation backend be
          available and texinfo don't be the one set as default.
    
        - When documentation manuals be created, the `manual.conf'
          configuration file will be created as part of the creation
          process. The `centos-art.sh' script uses the value of
          `manual_backend' variable, inside the `manual.conf' file to
          determine the documentation backend value to use.
    
        - Each documentation backend implemented must have its own
          `manual.conf' configuration file to set the `manual_backend'
          variable correctly.