Hello Wilson! I’m glad that you found my writing helpful. 
You are right, at the moment, dbt docs generate can only take into account one target. This is because dbt docs is only able to parse the project in one environment. I would be curious to ask what your use case would be for all of the targets. Do you picture in the docs, when you select a model, you see a list of all of the objects related to that dbt model, regardless of the environment you generated the docs from?