database/sql: document non-open of Open; add Ping
database/sql: document non-open of Open; add Ping

Fixes issue 4804

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/7819043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang