improve pdo docs
improve pdo docs

rquadling/phinx,aCodify/phinx,daniel-gomes-sociomantic/phinx,dankempster/phinx,ikagawa/phinx,courtney-miles/phinx,robmorgan/phinx,laurin1/phinx,BambooHR/phinx-mt,aCodify/phinx,Thamrongsak/phinx,DeltaPHP/phinx