Add more behavior change items

Author: Nick Pentreath

docs/ml-guide.md

@@ -133,6 +133,14 @@ new [`OneHotEncoderEstimator`](ml-features.html#onehotencoderestimator)
 * [SPARK-22156](https://issues.apache.org/jira/browse/SPARK-22156):
  The learning rate update for `Word2Vec` was incorrect when `numIterations` was set greater than
  `1`. This will cause training results to be different between `2.3` and earlier versions.
+* [SPARK-21681](https://issues.apache.org/jira/browse/SPARK-21681):
+ Fixed an edge case bug in multinomial logistic regression that resulted in incorrect coefficients
+ when some features had zero variance.
+* [SPARK-16957](https://issues.apache.org/jira/browse/SPARK-16957):
+ Tree algorithms now use mid-points for split values. This may change results from model training.
+* [SPARK-14657](https://issues.apache.org/jira/browse/SPARK-14657):
+ Fixed an issue where the features generated by `RFormula` without an intercept were inconsistent
+ with the output in R. This may change results from model training in this scenario.
 
 ## Previous Spark versions