no negative team id in docs

docs/Training-Self-Play.md

 
 ![Team ID](images/team_id.png)
 
-See the trainer configuration and agent prefabs for our Tennis environment for an example.
+***Team ID must be 0 or an integer greater than 0. Negative numbers will cause unpredictable behavior.*** See the trainer configuration and agent prefabs for our Tennis environment for an example.
 
 ## Best Practices Training with Self-Play