[SPARK-19006][DOCS] mention spark.kryoserializer.buffer.max must be less than 2048m in doc

## What changes were proposed in this pull request?

On configuration doc page:https://spark.apache.org/docs/latest/configuration.html
We mentioned spark.kryoserializer.buffer.max : Maximum allowable size of Kryo serialization buffer. This must be larger than any object you attempt to serialize. Increase this if you get a "buffer limit exceeded" exception inside Kryo.
from source code, it has hard coded upper limit :
```
val maxBufferSizeMb = conf.getSizeAsMb("spark.kryoserializer.buffer.max", "64m").toInt
if (maxBufferSizeMb >= ByteUnit.GiB.toMiB(2))
{ throw new IllegalArgumentException("spark.kryoserializer.buffer.max must be less than " + s"2048 mb, got: + $maxBufferSizeMb mb.") }
```
We should mention "this value must be less than 2048 mb" on the configuration doc page as well.

## How was this patch tested?

None. Since it's minor doc change.

Author: Yuexin Zhang <yxzhang@cloudera.com>

Closes #16412 from cnZach/SPARK-19006.

docs/configuration.md

@@ -840,8 +840,8 @@ Apart from these, the following properties are also available, and may be useful
   <td>64m</td>
   <td>
     Maximum allowable size of Kryo serialization buffer. This must be larger than any
-    object you attempt to serialize. Increase this if you get a "buffer limit exceeded" exception
-    inside Kryo.
+    object you attempt to serialize and must be less than 2048m.
+    Increase this if you get a "buffer limit exceeded" exception inside Kryo.
   </td>
 </tr>
 <tr>