Document how to set a default schema for pg and neon adapters.

[SevInf]

When adapters are used, DB connection is configured even before prisma client is created. schema query parameter is non-standard and is not recognized by JS drivers. We have a different way of configuring default schema with adapters:

const adapter = new PrismaPg(pool, {
  schema: 'myDbSchema'
})

this applies only to pg, neon and pg-worker adapters. We need to document this.

[nikolasburk]

Done via: https://github.com/prisma/docs/pull/5954/files