CL 21663 allowed drivers to implement ExecerContext without
also implementing Execer, and similarly QueryerContext without
Queryer, but it did not make that clear in the documentation.

This CL updates the documentation.

Change-Id: I9a4accaac32edfe255fe7c0b0907d4c1014322b4
Reviewed-on: https://go-review.googlesource.com/78129Reviewed-by: Daniel Theophanes <kardianos@gmail.com>