Update `help' functionality:
    
        - Update comments.
    
        - Move MANUAL_TEMPLATE variable definitions from `help.sh' file to
          `Texinfo/texinfo.sh' file. Different backend implementations
          might use different template definitions. In this case, this
          definitions are specific to texinfo documentation backend.
    
        - Rename variable name from DOCENTRY to MANUAL_DOCENTRY inside
          `help_getEntries.sh' file.