=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-docs/rst/java/code/docs/stream/javadsl/cookbook/RecipeParseLines.java

@@ -50,8 +50,7 @@ public class RecipeParseLines extends RecipeTest {
       .via(Framing.delimiter(ByteString.fromString("\r\n"), 100, true))
       .map(b -> b.utf8String());
     //#parse-lines
-
-    lines.grouped(10).runWith(Sink.head(), mat).toCompletableFuture().get(1, TimeUnit.SECONDS);
+    lines.limit(10).runWith(Sink.seq(), mat).toCompletableFuture().get(1, TimeUnit.SECONDS);
   }
 
 }