code: def _format_envvar(param):
      
      yield '.. envvar:: {}'.format(param.envvar)
      yield '   :noindex:'
      yield ''
      if isinstance(param, click.Argument):
          param_ref = param.human_readable_name
      else:
          # if a user has defined an opt with multiple "aliases", always use the
          # first. For example, if '--foo' or '-f' are possible, use '--foo'.
          param_ref = param.opts[0]
  
      yield _indent('Provide a default for :option:`{}`'.format(param_ref))
docs: Format the envvars of a `click.Option` or `click.Argument`.