Merge pull request #2472 from Unity-Technologies/release-0.9.2-multi-gpu-doc

Added the doc for multi-gpu

docs/Training-ML-Agents.md

   [here](https://docs.unity3d.com/Manual/CommandLineArguments.html) for more
   details.
 * `--debug`: Specify this option to enable debug-level logging for some parts of the code.
+* `--multi-gpu`: Setting this flag enables the use of multiple GPU's (if available) during training.
 
 ### Training Config File
 

ml-agents/mlagents/trainers/learn.py

       --docker-target-name=<dt>   Docker volume to store training-specific files [default: None].
       --no-graphics               Whether to run the environment in no-graphics mode [default: False].
       --debug                     Whether to run ML-Agents in debug mode with detailed logging [default: False].
-      --multi-gpu                Whether to use multiple GPU training [default: False].
+      --multi-gpu                 Setting this flag enables the use of multiple GPU's (if available) during training [default: False].
     """
 
     options = docopt(_USAGE)