Document and add tests for service autowiring

Problem/Motivation

The Symfony 3 container has the ability to autowire services in the container. Service autowiring is already supported by Drupal's container, but lacks test coverage and documentation.

Proposed resolution

Add test coverage for service autowiring, and document how to use this feature.

Remaining tasks

• Add test coverage
• Add documentation to core
• Review & commit
• Update online service docs
• https://www.drupal.org/docs/8/api/services-and-dependency-injection/serv...
• https://www.drupal.org/docs/8/api/services-and-dependency-injection/stru...

User interface changes

None

API changes

None

Data model changes

None

Release notes snippet

Add documentation and test coverage for service