SPARK-25004: Update configuration docs.

Author: rdblue

docs/configuration.md

@@ -185,7 +185,10 @@ of the most common options to set are:
   <td>
     The amount of memory to be allocated to PySpark in each executor, in MiB
     unless otherwise specified.  If set, PySpark memory for an executor will be
-    limited to this amount. If not set, Spark will not limit Python's memory use.
+    limited to this amount. If not set, Spark will not limit Python's memory use
+    and it is up to the application to avoid exceeding the overhead memory space
+    shared with other non-JVM processes. When PySpark is run in YARN, this memory
+    is added to executor resource requests.
   </td>
 </tr>
 <tr>