sql: document that for drivers, io.EOF means no more rows
sql: document that for drivers, io.EOF means no more rows

This was used in the sql package + tests, but never
documented.

R=golang-dev, gri
CC=golang-dev
http://codereview.appspot.com/5372107
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go