[REQ] Better document ability for config.yaml "!include"s to be within a child key

Created by: dragorosson

Is your feature request related to a problem? Please describe.

This is such a great feature! The documentation at https://openapi-generator.tech/docs/usage/#batch reads

Starting with 5.0.0, the !batch command supports multiple !include properties, either sequential or nested.

I took this to mean "an !included file can use !include itself". I didn't realize that you can use it nested within a key:

additionalProperties:
  '!include': commonProperties.yaml
  foo: bar

I had to look at the source to realize this. https://github.com/OpenAPITools/openapi-generator/blob/9be92fabd1e223d96205c8f19bdb426b3e9c608e/modules/openapi-generator-cli/src/main/java/org/openapitools/codegen/cmd/GenerateBatch.java#L349

If the example that uses !include had this usage, it would be clearer.