Also add a link to more information about isolation levels as defined by the
SQL standard. Fixes #17682.

Change-Id: I94c53b713f4c882af40cf15fe5f1e5dbc53ea741
Reviewed-on: https://go-review.googlesource.com/32418Reviewed-by: Daniel Theophanes <kardianos@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>