[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The spark.kryoserializer.buffer.max.mb is out-of-date in spark 1.4.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 #7393 from stanzhai/master and squashes the following commits:

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