[SPARK-9327] [DOCS] Fix documentation about classpath config options.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #7651 from vanzin/SPARK-9327 and squashes the following commits:

2923e23 [Marcelo Vanzin] [SPARK-9327] [docs] Fix documentation about classpath config options.

docs/configuration.md

@@ -203,7 +203,7 @@ Apart from these, the following properties are also available, and may be useful
   <td><code>spark.driver.extraClassPath</code></td>
   <td>(none)</td>
   <td>
-    Extra classpath entries to append to the classpath of the driver.
+    Extra classpath entries to prepend to the classpath of the driver.
 
     <br /><em>Note:</em> In client mode, this config must not be set through the <code>SparkConf</code>
     directly in your application, because the driver JVM has already started at that point.
@@ -250,7 +250,7 @@ Apart from these, the following properties are also available, and may be useful
   <td><code>spark.executor.extraClassPath</code></td>
   <td>(none)</td>
   <td>
-    Extra classpath entries to append to the classpath of executors. This exists primarily for 
+    Extra classpath entries to prepend to the classpath of executors. This exists primarily for 
     backwards-compatibility with older versions of Spark. Users typically should not need to set 
     this option.
   </td>