Documentation updates: Added Unity instance parallelization to enviro… (#5143)

* Updated Learning-Environment-Create-New.md with a section on parallel unity instances.

* Added trailing whitespace to Learning Environment Create New md file.

* Added trailing whitespace to Learning Environment Create New md file after fixes.

* Minor updates.

* Minor updates.

* Whitespace fixes.

docs/Learning-Environment-Create-New.md

    your Assets browser, turning it into a prefab.
 1. You can now instantiate copies of the TrainingArea prefab. Drag them into
    your scene, positioning them so that they do not overlap.
+
+## Optional: Training Using Concurrent Unity Instances
+Another level of parallelization comes by training using
+[concurrent Unity instances](ML-Agents-Overview.md#additional-features).
+For example,
+
+```
+mlagents-learn config/rollerball_config.yaml --run-id=RollerBall --num-envs=2
+```
+
+will start ML Agents training with two environment instances. Combining multiple
+training areas within the same scene, with concurrent Unity instances, effectively
+gives you two levels of parallelism to speed up training. The command line option
+`--num-envs=<n>` controls the number of concurrent Unity instances that are
+executed in parallel during training.