MINOR: Add explanation for disabling forwarding from value transformers

streams/src/main/java/org/apache/kafka/streams/processor/internals/ForwardingDisabledProcessorContext.java

@@ -40,6 +40,11 @@
 public final class ForwardingDisabledProcessorContext implements ProcessorContext {
     private final ProcessorContext delegate;
 
+    private static final String EXPLANATION = "ProcessorContext#forward() is not supported from this context, "
+        + "as the framework must ensure the key is not changed (#forward allows changing the key on "
+        + "messages which are sent). Try another function, which doesn't allow the key to be changed "
+        + "(for example - #tranformValues).";
+
     public ForwardingDisabledProcessorContext(final ProcessorContext delegate) {
         this.delegate = Objects.requireNonNull(delegate, "delegate");
     }
@@ -102,24 +107,24 @@ public Cancellable schedule(final Duration interval,
 
     @Override
     public <K, V> void forward(final K key, final V value) {
-        throw new StreamsException("ProcessorContext#forward() not supported.");
+        throw new StreamsException(EXPLANATION);
     }
 
     @Override
     public <K, V> void forward(final K key, final V value, final To to) {
-        throw new StreamsException("ProcessorContext#forward() not supported.");
+        throw new StreamsException(EXPLANATION);
     }
 
     @Override
     @Deprecated
     public <K, V> void forward(final K key, final V value, final int childIndex) {
-        throw new StreamsException("ProcessorContext#forward() not supported.");
+        throw new StreamsException(EXPLANATION);
     }
 
     @Override
     @Deprecated
     public <K, V> void forward(final K key, final V value, final String childName) {
-        throw new StreamsException("ProcessorContext#forward() not supported.");
+        throw new StreamsException(EXPLANATION);
     }
 
     @Override