Updated documentation for driver-memory to reflect its true behavior in client vs cluster mode

Author: Ilya Ganelin

docs/configuration.md

@@ -115,6 +115,8 @@ of the most common options to set are:
   <td>
     Amount of memory to use for the driver process, i.e. where SparkContext is initialized.
     (e.g. <code>512m</code>, <code>2g</code>).
+    
+    <br /><br />Note: this setting only works in <code>cluster</code> mode (e.g. YARN deployment). In client mode (e.g. spark-shell), this setting has no effect. In client mode, driver memory should be configured in the run-time settings ; i.e. --driver-memory 2g.
   </td>
 </tr>
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