-1 for length in mysql_stmt_prepare isn't documented

Using -1 for the third, length parameter of mysql_stmt_prepare seems to work and is also used in the sample on this page:
https://mariadb.com/kb/en/mariadb/bulk-insert-row-wise-binding/
But it is not documented here:
https://mariadb.com/kb/en/mariadb/mysql_stmt_prepare/

I don't see MySQL documenting -1 as a valid value either, so either we document this as valid (which can potentially break MySQL compatibility) or we don't use it in the samples.