[SPARK-4884]: Improve Partition docs

Rewording was based on this discussion: http://apache-spark-developers-list.1001551.n3.nabble.com/RDD-data-flow-td9804.html
This is the associated JIRA ticket: https://issues.apache.org/jira/browse/SPARK-4884

Author: Madhu Siddalingaiah <madhu@madhu.com>

Closes #3722 from msiddalingaiah/master and squashes the following commits:

79e679f [Madhu Siddalingaiah] [DOC]: improve documentation
51d14b9 [Madhu Siddalingaiah] Merge remote-tracking branch 'upstream/master'
38faca4 [Madhu Siddalingaiah] Merge remote-tracking branch 'upstream/master'
cbccbfe [Madhu Siddalingaiah] Documentation: replace <b> with <code> (again)
332f7a2 [Madhu Siddalingaiah] Documentation: replace <b> with <code>
cd2b05a [Madhu Siddalingaiah] Merge remote-tracking branch 'upstream/master'
0fc12d7 [Madhu Siddalingaiah] Documentation: add description for repartitionAndSortWithinPartitions

Author: msiddalingaiah

core/src/main/scala/org/apache/spark/Partition.scala

@@ -18,11 +18,11 @@
 package org.apache.spark
 
 /**
- * A partition of an RDD.
+ * An identifier for a partition in an RDD.
  */
 trait Partition extends Serializable {
   /**
-   * Get the split's index within its parent RDD
+   * Get the partition's index within its parent RDD
    */
   def index: Int