database/sql: more docs explaining that DB is a pool
database/sql: more docs explaining that DB is a pool

This is the main point of confusion and the emphasis of
a recent Gophercon talk.

Fixes Issue 5886. (mostly fixed in previous commits)

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/100560043

firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang