Make pgSettings clearer

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

Open src/pages/postgraphile/usage-library.md and read the pgSettings function section. Add examples showing period-less settings such as role and statement_timeout, alongside dotted settings such as jwt.claims.user_id. Done means the page clearly distinguishes settings passed to PostgreSQL from settings intended for application code.

Written by the indexing model from the issue text.