Document `Memcached::__construct()` parameters

[cracksalad]

This is a follow up of #3873

The callback and connection_str parameters are not yet documented.

https://github.com/php/doc-en/blob/54eaf4ed9edc37074870a13dc0de605df4285c3b/reference/memcached/memcached/construct.xml#L40-L55