Inconsistent documentation on named types for custom mutations

It's amazing that postgraphile can now handle all sorts of functions. The documentation is updated at https://www.graphile.org/postgraphile/function-restrictions/, but not at https://www.graphile.org/postgraphile/custom-mutations/. You should probably update the latter as well to avoid any confusion :)

Read the guidance at https://www.graphile.org/postgraphile/function-restrictions/ and compare it with https://www.graphile.org/postgraphile/custom-mutations/. Update the custom mutations documentation so its named-type guidance is consistent with the function restrictions page, then verify the published page is no longer confusing.

索引モデルが issue の本文から書いたものです。