[hotfix] Added missing documentation in StateTransformationFunction

flink-runtime/src/main/java/org/apache/flink/runtime/state/StateTransformationFunction.java

@@ -18,6 +18,25 @@
 
 package org.apache.flink.runtime.state;
 
+import org.apache.flink.annotation.Internal;
+
+/**
+ * Interface for a binary function that is used for push-down of state transformation into state backends. The
+ * function takes as inputs the old state and an element. From those inputs, the function computes the new state.
+ *
+ * @param <S> type of the previous state that is the bases for the computation of the new state.
+ * @param <T> type of the element value that is used to compute the change of state.
+ */
+@Internal
 public interface StateTransformationFunction<S, T> {
+
+	/**
+	 * Binary function that applies a given value to the given old state to compute the new state.
+	 *
+	 * @param previousState the previous state that is the basis for the transformation.
+	 * @param value         the value that the implementation applies to the old state to obtain the new state.
+	 * @return the new state, computed by applying the given value on the given old state.
+	 * @throws Exception if something goes wrong in applying the transformation function.
+	 */
 	S apply(S previousState, T value) throws Exception;
-}
+}