Document how to use schema_mapping setting on transferwise variants of target-snowflake and target-postgres

The format of the setting is not currently documented:

• https://meltano.com/plugins/loaders/snowflake--transferwise.html#schema-mapping
• https://meltano.com/plugins/loaders/postgres--transferwise.html#schema-mapping

Reading the PipelineWise source reveals the intended format:

{
  "<source_schema>": {
    "target_schema": "<target_schema>",
    "target_schema_select_permissions": "<target_schema_select_permissions>",
  },
  # ...
}

These <variables> correspond to the values in the PipelineWise YAML config that can be seen in #2385 (closed):

schemas:

  - source_schema: "public"            # Source schema in postgres with tables
    target_schema: "repl_pg_public"    # Target schema in the destination Data Warehouse
    target_schema_select_permissions:  # Optional: Grant SELECT on schema and tables that created
      - grp_stats

  # Here go `tables`

  # You can add as many schemas as you need...
  # Uncomment this if you want replicate tables from multiple schemas
  #- source_schema: "another_schema_in_postgres"
  #  target_schema: "another