Generate an explain command document.

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)])