=str #19293 fix issues in Sink.seq + minor doc fixes + use Sink.seq and limit in tests where appropriate

* Sink.seq (Scala DSL) now returns immutable.Seq rather than Seq
* Sink.seq will not silently truncate when incoming elements is > Int.MAX_VALUE
* minor doc fixes
* replacing various grouped(n) / Sink.head with limit(n) / Sink.seq in various tests
* fix inconsistent indentation in RequestParserSpec

akka-stream-tests/src/test/java/akka/stream/javadsl/SourceTest.java

@@ -321,7 +321,7 @@ public class SourceTest extends StreamTest {
     Pair<List<Integer>, Source<Integer, NotUsed>> result = future.toCompletableFuture().get(3, TimeUnit.SECONDS);
     assertEquals(Arrays.asList(1, 2, 3), result.first());
 
-    CompletionStage<List<Integer>> tailFuture = result.second().grouped(4).runWith(Sink.<List<Integer>>head(), materializer);
+    CompletionStage<List<Integer>> tailFuture = result.second().limit(4).runWith(Sink.<Integer>seq(), materializer);
     List<Integer> tailResult = tailFuture.toCompletableFuture().get(3, TimeUnit.SECONDS);
     assertEquals(Arrays.asList(4, 5, 6), tailResult);
   }