Add links to instructions for calling computed column functions _outside_ of PostGraphile

[benjie]

If someone wants to debug a computed column function it's not necessarily obvious how to call it; see: #2331 (comment)

I added https://github.com/graphile/crystal/pull/2337/files to demonstrate, but this should be cross-linked in other places in the documentation:

an explanation of how to debug computed columns would be valuable on postgraphile.org/postgraphile/next/computed-columns and/or postgraphile.org/postgraphile/next/debugging!

Perhaps add a heading to postgraphile.org/postgraphile/next/computed-columns just above "PostgreSQL's documentation on function calls notes:" so that you can cross-link more easily?