docs: def _gen_explain_command(
          coll, spec, projection, skip, limit, batch_size,
          options, read_concern):
      """Generate an explain command document."""
      cmd = _gen_find_command(
          coll, spec, projection, skip, limit, batch_size, options)
      if read_concern.level:
          return SON([('explain', cmd), ('readConcern', read_concern.document)])
      return SON([('explain', cmd)])