Additional documentation for yarn memory overhead issue

Author: nishkamravi2

yarn/common/src/main/scala/org/apache/spark/deploy/yarn/YarnSparkHadoopUtil.scala

@@ -85,6 +85,9 @@ class YarnSparkHadoopUtil extends SparkHadoopUtil {
 
 object YarnSparkHadoopUtil {
   // Additional memory overhead 
+  // 7% was arrived at experimentally. In the interest of minimizing memory waste while covering
+  // the common cases. Memory overhead tends to grow with container size. 
+
   val MEMORY_OVERHEAD_FACTOR = 0.07
   val MEMORY_OVERHEAD_MIN = 384