This aims to expand the coverage of examples showing how the sql
package works, as well as to address a number of issues I've observed
while explaining how the database package works:

- The best way to issue UPDATE or INSERT queries, that don't need
to scan anything in return. (Previously, we had no examples for any
Execute statement).

- How to use prepared statements and transactions.

- How to aggregate arguments from a Query/QueryContext query into
a slice.

Furthermore just having examples in more places should help, as users
click on e.g. the "Rows" return parameter and are treated with the
lack of any example about how Rows is used.

Switch package examples to use QueryContext/QueryRowContext; I think
it is a good practice to prepare users to issue queries with a timeout
attached, even if they are not using it immediately.

Change-Id: I4e63af91c7e4fff88b25f820906104ecefde4cc3
Reviewed-on: https://go-review.googlesource.com/91015Reviewed-by: Daniel Theophanes <kardianos@gmail.com>
Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>