Describe Docker authorization config precedence

With %v1.8 we've improved the support for private Docker registries. We've updated the documentation, however it doesn't mention the precedence of authorization credentials resolving:

1. Search for credentials for the image from DOCKER_AUTH_CONFIG variable. If not present, then
2. Search for credentials for the image from ~/.docker/config.json or ~/.dockercfg files on Runner's host. If not present, then
3. Search for credentials for the image in data send with job's payload.

We should add this to the documentation.