The driver.Value type may be more then the documented 6 types if the
database driver supports it. Document that fact.

Updates #23077

Change-Id: If7e2112fa61a8cc4e155bb31e94e89b20c607242
Reviewed-on: https://go-review.googlesource.com/84636Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>