[cache] document the hash_algo option

[dbu]

https://github.com/php-http/cache-plugin/blob/b2f15cee4ff46f61f7b90137b41627d17fd57be9/src/CachePlugin.php#L55 is not in http://docs.php-http.org/en/latest/plugins/cache.html

we should try to be a bit more verbose in the doc about what the algo means.