[SPARK-9010] [DOCUMENTATION] Improve the Spark Configuration document…

… about `spark.kryoserializer.buffer`

The meaning of spark.kryoserializer.buffer should be "Initial size of Kryo's serialization buffer. Note that there will be one buffer per core on each worker. This buffer will grow up to spark.kryoserializer.buffer.max if needed.".

The spark.kryoserializer.buffer.max.mb is out-of-date in spark 1.4.

Author: zhaishidan <zhaishidan@haizhi.com>

Closes apache#7393 from stanzhai/master and squashes the following commits:

69729ef [zhaishidan] fix document error about spark.kryoserializer.buffer.max.mb

docs/configuration.md

@@ -665,7 +665,7 @@ Apart from these, the following properties are also available, and may be useful
   <td>
     Initial size of Kryo's serialization buffer. Note that there will be one buffer
      <i>per core</i> on each worker. This buffer will grow up to
-     <code>spark.kryoserializer.buffer.max.mb</code> if needed.
+     <code>spark.kryoserializer.buffer.max</code> if needed.
   </td>
 </tr>
 <tr>