Address PR comments.

Author: lukecwik

model/pipeline/src/main/proto/beam_runner_api.proto

@@ -205,8 +205,8 @@ message StandardPTransforms {
     // Represents mapping of main input window onto side input window.
     //
     // Side input window mapping function:
-    // Input: KV<nonce, SideInputWindow>
-    // Output: KV<nonce, MainInputWindow>
+    // Input: KV<nonce, MainInputWindow>
+    // Output: KV<nonce, SideInputWindow>
     //
     // For each main input window, the side input window is returned. The
     // nonce is used by a runner to associate each input with its output.

sdks/java/harness/src/main/java/org/apache/beam/fn/harness/WindowMappingFnRunner.java

@@ -37,8 +37,8 @@
  * <p>Side input window mapping function:
  *
  * <ul>
- *   <li>Input: {@code KV<nonce, SideInputWindow>}
- *   <li>Output: {@code KV<nonce, MainInputWindow>}
+ *   <li>Input: {@code KV<nonce, MainInputWindow>}
+ *   <li>Output: {@code KV<nonce, SideInputWindow>}
  * </ul>
  *
  * <p>For each main input window, the side input window is returned. The nonce is used by a runner

sdks/java/harness/src/main/java/org/apache/beam/fn/harness/WindowMergingFnRunner.java

@@ -87,7 +87,7 @@ static <T, W extends BoundedWindow> WindowMergingFnRunner<T, W> create(WindowFn<
     if (windowFn.isNonMerging()) {
       return new NonMergingWindowFnRunner();
     } else {
-      return new MergingWindowFnRunner(windowFn);
+      return new MergingViaWindowFnRunner(windowFn);
     }
   }
 
@@ -118,14 +118,14 @@ KV<T, KV<Iterable<W>, Iterable<KV<W, Iterable<W>>>>> mergeWindows(
   }
 
   /** An implementation which uses a {@link WindowFn} to merge windows. */
-  private static class MergingWindowFnRunner<T, W extends BoundedWindow>
+  private static class MergingViaWindowFnRunner<T, W extends BoundedWindow>
       extends WindowMergingFnRunner<T, W> {
     private final WindowFn<?, W> windowFn;
     private final WindowFn<?, W>.MergeContext mergeContext;
     private Collection<W> currentWindows;
     private List<KV<W, Collection<W>>> mergedWindows;
 
-    private MergingWindowFnRunner(WindowFn<?, W> windowFn) {
+    private MergingViaWindowFnRunner(WindowFn<?, W> windowFn) {
       this.windowFn = windowFn;
       this.mergedWindows = new ArrayList<>();
       this.currentWindows = new ArrayList<>();