Document AI is introducing document-level prompting for custom document

## Feature Feature Document AI is introducing document-level prompting for custom document processors. This feature allows you to provide an overall description of the document to inject deep business knowledge into the model, leading to improved extraction quality. Document-level prompting offers improved accuracy by giving the model necessary context for extraction at the document level. This allows for easily supplied general information, such as geographical limitations (for example, _all the address fields are located in the USA_), to guide the model. For more details, refer to the documentation on custom extractor mechanisms and [document-level prompting](https://docs.cloud.google.com/document-ai/docs/ce-mechanisms#document-level-prompting).