Make pgSettings clearer

[benjie]

e.g. demonstrate settings you want to be period-less (e.g. role, statement_timeout which affect PostgreSQL directly) and settings you want to have periods in (e.g. jwt.claims.user_id etc which don't affect PostgreSQL directly but your own application code can use).

This page: https://www.graphile.org/postgraphile/usage-library/#pgsettings-function

This file: https://github.com/graphile/graphile.github.io/blob/develop/src/pages/postgraphile/usage-library.md