[SPARK-29924][DOCS] Add Apache Arrow requirement in Java 11

Author: dongjoon-hyun

docs/index.md

@@ -52,6 +52,8 @@ For the Scala API, Spark {{site.SPARK_VERSION}}
 uses Scala {{site.SCALA_BINARY_VERSION}}. You will need to use a compatible Scala version
 ({{site.SCALA_BINARY_VERSION}}.x).
 
+For Java 11, `-Dio.netty.tryReflectionSetAccessible=true` is required additionally for Apache Arrow library. This prevents `java.lang.UnsupportedOperationException: sun.misc.Unsafe or java.nio.DirectByteBuffer.(long, int) not available` when Apache Arrow uses Netty internally.
+
 # Running the Examples and Shell
 
 Spark comes with several sample programs.  Scala, Java, Python and R examples are in the