Add pooled client docstring
Add pooled client docstring

ewdurbin/pymemcache,dive-tv/pyelasticache_client,sontek/pymemcache,thedrow/pymemcache,mbrukman/pymemcache,duanhongyi/pymemcache,pinterest/pymemcache,sontek/pymemcache,bwalks/pymemcache,harlowja/pymemcache,adamchainz/pymemcache,pinterest/pymemcache