[CSL-1165] Actualize haddock comment for Pos.DB.Class module
[CSL-1165] Actualize haddock comment for Pos.DB.Class module

input-output-hk/pos-haskell-prototype,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/cardano-sl