Add note about spark.executor.heartbeatInterval change to migration guide

Author: srowen

docs/sql-migration-guide-upgrade.md

@@ -124,6 +124,14 @@ license: |
 
   - In Spark version 2.4, when a spark session is created via `cloneSession()`, the newly created spark session inherits its configuration from its parent `SparkContext` even though the same configuration may exist with a different value in its parent spark session. Since Spark 3.0, the configurations of a parent `SparkSession` have a higher precedence over the parent `SparkContext`.
 
+## Upgrading from Spark SQL 2.4 to 2.4.1
+
+  - The value of `spark.executor.heartbeatInterval`, when specified without units like "30" rather than "30s", was
+    inconsistently interpreted as both seconds and milliseconds in Spark 2.4.0 in different parts of the code.
+    Unitless values are now consistently interpreted as milliseconds. Applications that set values like "30"
+    need to specify a value with units like "30s" now, to avoid being interpreted as milliseconds; otherwise, 
+    the extremely short interval that results will likely cause applications to fail.
+
 ## Upgrading From Spark SQL 2.3 to 2.4
 
   - In Spark version 2.3 and earlier, the second parameter to array_contains function is implicitly promoted to the element type of first array type parameter. This type promotion can be lossy and may cause `array_contains` function to return wrong result. This problem has been addressed in 2.4 by employing a safer type promotion mechanism. This can cause some change in behavior and are illustrated in the table below.