Add inline comment.

Author: ueshin

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/CodeGenerator.scala

@@ -164,6 +164,8 @@ class CodegenContext {
     // It's possible that we add same mutable state twice, e.g. the `mergeExpressions` in
     // `TypedAggregateExpression`, we should call `distinct` here to remove the duplicated ones.
     val initCodes = mutableStates.distinct.map(_._3 + "\n")
+    // The generated initialization code may exceed 64kb function size limit in JVM if there are too
+    // many mutable states, so split it into multiple functions.
     splitExpressions(initCodes, "init", Nil)
   }