Add removal of multi-gpu to changelog and migrating

com.unity.ml-agents/CHANGELOG.md

  - Agent.CollectObservations now takes a VectorSensor argument. It was also overloaded to optionally take an ActionMasker argument. (#3352, #3389)
  - Beta support for ONNX export was added. If the `tf2onnx` python package is installed, models will be saved to `.onnx` as well as `.nn` format.
  Note that Barracuda 0.6.0 or later is required to import the `.onnx` files properly
+ - Multi-GPU training and the `--multi-gpu` option has been removed temporarily. (#3345)
 
 ### Minor Changes
  - Monitor.cs was moved to Examples. (#3372)

docs/Migrating.md

 * The `MLAgents.Sensor` namespace has been removed. All sensors now belong to the `MLAgents` namespace.
 * The `SetActionMask` method must now be called on the optional `ActionMasker` argument of the `CollectObservations` method. (We now consider an action mask as a type of observation)
 * The method `GetStepCount()` on the Agent class has been replaced with the property getter `StepCount`
+* The `--multi-gpu` option has been removed temporarily.
 
 ### Steps to Migrate
 * Replace your Agent's implementation of `CollectObservations()` with `CollectObservations(VectorSensor sensor)`. In addition, replace all calls to `AddVectorObs()` with `sensor.AddObservation()` or `sensor.AddOneHotObservation()` on the `VectorSensor` passed as argument.