docs: def upgrade(*pkgs):
      '''
      Runs an update operation on the specified packages, or all packages if none is specified.
  
      :type pkgs: list(str)
      :param pkgs:
          List of packages to update
  
      :return: The upgraded packages. Example element: ``['libxslt-1.1.0', 'libxslt-1.1.10']``
      :rtype: list(tuple(str, str))
  
      .. code-block:: bash
  
          salt '*' nix.update
          salt '*' nix.update pkgs=one,two
      '''
      cmd = _quietnix()
      cmd.append('--upgrade')
      cmd.extend(pkgs)
  
      out = _run(cmd)
  
      upgrades = [_format_upgrade(s.split(maxsplit=1)[1])
                  for s in out['stderr'].splitlines()
                  if s.startswith('upgrading')]
  
      return [[_strip_quotes(s_) for s_ in s]
              for s in upgrades]