Update `help' functionality:
    
        - Start using `help_texinfo' suffix on files to identify them as
          files that manipulate the repository documentation manual
          in Texinfo format. This is required to avoid confusion with
          other files that manipulate the repository documentation manual
          in other formats (e.g., Docbook, LinuxDoc, etc.)
    
        - Add the `--format' option (FLAG_FORMAT) to control the backend
          format used to manipulate the repository documentation manual.
          By default Docbook is used.