I was wondering if there is some kind of documentation available for Google's AutoML Vision to train recognizing specific logo's.
At this point I only can find documentation about object detection.
Google already has a feature to detect popular product logos within an image - Detect logos. However it's managed by google and you won't find all logos there. It will recognize worldwide/know logos like GENERAL ELECTRIC, Shell, Nike.
There is also a IssueTracker - Feature Request which could give the possibility of modifying Vision API logo detection. You can star it to show you would also like this feature.
To achieve what you want, to recognise less known logos, you would need the AutoML train as you mention.
In AutoML Vision API Tutorial you can find some general information about training, however to your scenario I would suggest you to follow Building Image Detection with Google Cloud AutoML guide where Image Detection was used (open in private view in the browser if you have limitations).
You will find there all steps from Gathering Images through Image Labelling, Model Training, Model Evaluation to Outputs and Conclusions.
In this case you would need to use AutoML from scratch so you would need to train it using all logos.