cleanup: yamato triggers, changelog, docstrings

.yamato/com.unity.ml-agents-test.yml

     {% if platform.name == "linux" %}
     expression: |
       (pull_request.target eq "main" OR
-       pull_request.target eq "v2-staging" OR
       pull_request.target match "release.+") AND
       NOT pull_request.draft AND
       (pull_request.changes.any match "com.unity.ml-agents/**" OR

.yamato/compressed-sensor-test.yml

     {% if editor.extra_test == "sensor" %}
     expression: |
       (pull_request.target eq "main" OR
-       pull_request.target eq "v2-staging" OR
       pull_request.target match "release.+") AND
       NOT pull_request.draft AND
       (pull_request.changes.any match "com.unity.ml-agents/**" OR

.yamato/gym-interface-test.yml

     {% if editor.extra_test == "gym" %}
     expression: |
       (pull_request.target eq "main" OR
-       pull_request.target eq "v2-staging" OR
       pull_request.target match "release.+") AND
       NOT pull_request.draft AND
       (pull_request.changes.any match "com.unity.ml-agents/**" OR

.yamato/protobuf-generation-test.yml

     cancel_old_ci: true
     expression: |
       (pull_request.target eq "main" OR
-       pull_request.target eq "v2-staging" OR
       pull_request.target match "release.+") AND
       NOT pull_request.draft AND
       (pull_request.changes.any match "protobuf-definitions/**" OR

.yamato/python-ll-api-test.yml

     {% if editor.extra_test == "llapi" %}
     expression: |
       (pull_request.target eq "main" OR
-       pull_request.target eq "v2-staging" OR
       pull_request.target match "release.+") AND
       NOT pull_request.draft AND
       (pull_request.changes.any match "com.unity.ml-agents/**" OR

.yamato/standalone-build-test.yml

     cancel_old_ci: true
     expression: |
       (pull_request.target eq "main" OR
-       pull_request.target eq "v2-staging" OR
       pull_request.target match "release.+") AND
       NOT pull_request.draft AND
       (pull_request.changes.any match "com.unity.ml-agents/**" OR

.yamato/training-int-tests.yml

     cancel_old_ci: true
     expression: |
       (pull_request.target eq "main" OR
-       pull_request.target eq "v2-staging" OR
       pull_request.target match "release.+") AND
       NOT pull_request.draft AND
       (pull_request.changes.any match "com.unity.ml-agents/**" OR

com.unity.ml-agents/CHANGELOG.md

 ## [1.9.0-preview] - 2021-03-17
 ### Major Changes
 #### com.unity.ml-agents (C#)
-- The `BufferSensor` and `BufferSensorComponent` have been added. They allow the Agent to observe variable number of entities. (#4909)
-#### ml-agents / ml-agents-envs / gym-unity (Python)
-
-### Minor Changes
-#### com.unity.ml-agents / com.unity.ml-agents.extensions (C#)
-- Make com.unity.modules.unityanalytics an optional dependency. (#5109)
-#### ml-agents / ml-agents-envs / gym-unity (Python)
-
-### Bug Fixes
-#### com.unity.ml-agents / com.unity.ml-agents.extensions (C#)
-#### ml-agents / ml-agents-envs / gym-unity (Python)
-
-## [1.9.0-preview] - 2021-03-17
-### Major Changes
-#### com.unity.ml-agents (C#)
 - The `BufferSensor` and `BufferSensorComponent` have been added. They allow the Agent to observe variable number of entities. For an example, see the [Sorter environment](https://github.com/Unity-Technologies/ml-agents/blob/release_15_docs/docs/Learning-Environment-Examples.md#sorter). (#4909)
 - The `SimpleMultiAgentGroup` class and `IMultiAgentGroup` interface have been added. These allow Agents to be given rewards and
   end episodes in groups. For examples, see the [Cooperative Push Block](https://github.com/Unity-Technologies/ml-agents/blob/release_15_docs/docs/Learning-Environment-Examples.md#cooperative-push-block), [Dungeon Escape](https://github.com/Unity-Technologies/ml-agents/blob/release_15_docs/docs/Learning-Environment-Examples.md#dungeon-escape) and [Soccer](https://github.com/Unity-Technologies/ml-agents/blob/release_15_docs/docs/Learning-Environment-Examples.md#soccer-twos) environments. (#4923)
 #### com.unity.ml-agents / com.unity.ml-agents.extensions (C#)
 - Updated com.unity.barracuda to 1.3.2-preview. (#5084)
 - Added 3D Ball to the `com.unity.ml-agents` samples. (#5077)
+- Make com.unity.modules.unityanalytics an optional dependency. (#5109)
+
 #### ml-agents / ml-agents-envs / gym-unity (Python)
 - The `encoding_size` setting for RewardSignals has been deprecated. Please use `network_settings` instead. (#4982)
 - Sensor names are now passed through to `ObservationSpec.name`. (#5036)

com.unity.ml-agents/Runtime/Sensors/BufferSensor.cs

         int m_CurrentNumObservables;
         ObservationSpec m_ObservationSpec;
 
+
+        /// <summary>
+        /// Creates the BufferSensor.
+        /// </summary>
+        /// <param name="maxNumberObs">The maximum number of observations to be appended to this BufferSensor.</param>
+        /// <param name="obsSize">The size of each observation appended to the BufferSensor.</param>
+        /// <param name="name">The name of the sensor.</param>
         public BufferSensor(int maxNumberObs, int obsSize, string name)
         {
             m_Name = name;

com.unity.ml-agents/Runtime/Sensors/CameraSensor.cs

             return m_Name;
         }
 
-        /// <inheritdoc/>
+        /// <summary>
+        /// Returns a description of the observations that will be generated by the sensor.
+        /// The shape will be h x w x 1 for grayscale and h x w x 3 for color.
+        /// The dimensions have translational equivariance along width and height,
+        /// and no property along the channels dimension.
+        /// </summary>
+        /// <returns></returns>
         public ObservationSpec GetObservationSpec()
         {
             return m_ObservationSpec;