docs: def load_completions(self):
          """Load completions from the completion index.
  
          Updates the following attributes:
              * commands
              * subcommands
              * global_opts
              * args_opts
          """
          try:
              index_str = self.load_index(utils.AWSCLI_VERSION)
          except IndexLoadError:
              return
          index_str = self.load_index(utils.AWSCLI_VERSION)
          index_data = json.loads(index_str)
          index_root = index_data['aws']
          # ec2, s3, elb...
          self.commands = index_root['commands']
          # --profile, --region, --output...
          self.global_opts = index_root['arguments']
          for command in self.commands:
              # ec2: start-instances, stop-instances, terminate-instances...
              subcommands_current = index_root['children'] \
                  .get(command)['commands']
              self.subcommands.extend(subcommands_current)
              for subcommand_current in subcommands_current:
                  # start-instances: --instance-ids, --dry-run...
                  args_opts_current = index_root['children'] \
                      .get(command)['children'] \
                      .get(subcommand_current)['arguments']
                  self.args_opts.update(args_opts_current)