database/sql/driver: update Value doc, can be driver supported type

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>

src/database/sql/driver/driver.go

@@ -15,7 +15,8 @@ import (
 )
 
 // Value is a value that drivers must be able to handle.
-// It is either nil or an instance of one of these types:
+// It is either nil, a type handled by a database driver's NamedValueChecker
+// interface, or an instance of one of these types:
 //
 //   int64
 //   float64