[SPARK-29924][DOCS] Document Apache Arrow JDK11 requirement

### What changes were proposed in this pull request?

This adds a note for additional setting for Apache Arrow library for Java 11.

### Why are the changes needed?

Since Apache Arrow 0.14.0, an additional setting is required for Java 9+.
- https://issues.apache.org/jira/browse/ARROW-3191

It's explicitly documented at Apache Arrow 0.15.0.
- https://issues.apache.org/jira/browse/ARROW-6206

However, there is no plan to handle that inside Apache Arrow side.
- https://issues.apache.org/jira/browse/ARROW-7223

In short, we need to document this for the users who is using Arrow-related feature on JDK11.

For dev environment, we handle this via [SPARK-29923](#26552) .

### Does this PR introduce any user-facing change?

Yes.

### How was this patch tested?

Generated document and see the pages.

![doc](https://user-images.githubusercontent.com/9700541/73096611-0f409d80-3e9a-11ea-804b-c6b5ec7bd78d.png)

Closes #27356 from dongjoon-hyun/SPARK-JDK11-ARROW-DOC.

Authored-by: Dongjoon Hyun <dhyun@apple.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

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