Document need for `toFragment` when mapping multiple columns to one type

Hello,

I recently worked through a case similar to the Point example which describes the need for Read[A] and Write[A]. The select worked well, the insert/update failed with 'type mismatch' and it took some digging in gitter to learn that I needed:

case class Key(x: Int, y: Int)
case class Foo(key: Key, z: Int) // Need to map key to two columns x,y in the DB

// In my doobie code
fr"""insert into foo values (""" ++ keyWrite.toFragment(foo.key) ++ """, ${foo.z})""".update

Start with the existing Point example and the documentation around Read[A], Write[A], and toFragment. Add a clear example explaining how to map multiple database columns to one type during insert/update, and ensure the documentation makes the required toFragment usage discoverable.