[SPARK-12515][SQL][DOC] minor doc update for read.jdbc

Author: felixcheung <felixcheung_m@hotmail.com>

Closes #10465 from felixcheung/dfreaderjdbcdoc.

sql/core/src/main/scala/org/apache/spark/sql/DataFrameReader.scala

@@ -154,13 +154,14 @@ class DataFrameReader private[sql](sqlContext: SQLContext) extends Logging {
    * Don't create too many partitions in parallel on a large cluster; otherwise Spark might crash
    * your external database systems.
    *
-   * @param url JDBC database url of the form `jdbc:subprotocol:subname`
+   * @param url JDBC database url of the form `jdbc:subprotocol:subname`.
    * @param table Name of the table in the external database.
    * @param columnName the name of a column of integral type that will be used for partitioning.
-   * @param lowerBound the minimum value of `columnName` used to decide partition stride
-   * @param upperBound the maximum value of `columnName` used to decide partition stride
-   * @param numPartitions the number of partitions.  the range `minValue`-`maxValue` will be split
-   *                      evenly into this many partitions
+   * @param lowerBound the minimum value of `columnName` used to decide partition stride.
+   * @param upperBound the maximum value of `columnName` used to decide partition stride.
+   * @param numPartitions the number of partitions. This, along with `lowerBound` (inclusive),
+   *                      `upperBound` (exclusive), form partition strides for generated WHERE
+   *                      clause expressions used to split the column `columnName` evenly.
    * @param connectionProperties JDBC database connection arguments, a list of arbitrary string
    *                             tag/value. Normally at least a "user" and "password" property
    *                             should be included.