[DOCS] Updates infer trained model API docs with inference_config

[szabosteve]

Overview

This PR updates the infer trained model API docs by adding the inference_config property to the request body parameters. The parameter descriptions are single-sourced from the ml/ml-shared.asciidoc file.

It also adds the missing results_field to the inference_config property in the PUT API docs.

Preview

• Infer trained model API docs
• Create trained model API docs

[elasticmachine]

Pinging @elastic/es-docs (Team:Docs)

[elasticmachine]

Pinging @elastic/ml-core (Team:ML)

[benwtrent]

All the nlp inference configs also support a results_field parameter like classification and regression

[szabosteve]

@benwtrent Thanks for your review. I applied your suggestions and added results_field to the task configs. If I understand it correctly, the results_field should be present in the inference_config in PUT, too, shouldn't it?

[elasticsearchmachine]

💚 Backport successful

Status	Branch	Result
✅	8.3