Ensure that the `preset.grafast.context.pgSettings` example in the docs actually works
Have a vague feeling you need to define it via the `pgServices` section instead due to an assertion that prevents overwriting. We should check and either remove the assertion or fix the docs.

```ts
declare global {
  namespace Grafast {
    interface Context {
      user?: unknown
      snowflakeConnection?: unknown
    }
  }
}
const preset: GraphileConfig.Preset = {
  grafast: {
    explain: true,
    context(ctx) {
      const user = ctx.node?.req.user;
      return {
        user,
        snowflakeConnection: ctx.node?.req.snowflakeConnection
        pgSettings: {
          ...args.contextValue?.pgSettings,
          "jwt.claims.user_id": user?.id,
        }
      }
    },
  },
```

> ^ Note this is probably invalid currently - I think you actually have to configure the pgSettings via `pgServices: [makePgService({pgSettings(ctx){return {'jwt.claims.user_id': ctx.node?.req.user?.id}}})` call due to an over-cautious protection to prevent you accidentally overwriting pgSettings.