[SPARK-25261][MINOR][DOC] update the description for spark.executor|driver.memory in configuration.md

## What changes were proposed in this pull request?

As described in [SPARK-25261](https://issues.apache.org/jira/projects/SPARK/issues/SPARK-25261)，the unit of spark.executor.memory and spark.driver.memory is parsed as bytes in some cases if no unit specified, while in https://spark.apache.org/docs/latest/configuration.html#application-properties, they are descibed as MiB, which may lead to some misunderstandings.

## How was this patch tested?

N/A

Closes apache#22252 from ivoson/branch-correct-configuration.

Lead-authored-by: huangtengfei02 <[email protected]>
Co-authored-by: Huang Tengfei <[email protected]>
Signed-off-by: Sean Owen <[email protected]>

Author: 2 people

docs/configuration.md

@@ -152,8 +152,9 @@ of the most common options to set are:
   <td><code>spark.driver.memory</code></td>
   <td>1g</td>
   <td>
-    Amount of memory to use for the driver process, i.e. where SparkContext is initialized, in MiB 
-    unless otherwise specified (e.g. <code>1g</code>, <code>2g</code>).
+    Amount of memory to use for the driver process, i.e. where SparkContext is initialized, in the
+    same format as JVM memory strings with a size unit suffix ("k", "m", "g" or "t")
+    (e.g. <code>512m</code>, <code>2g</code>).
     <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.
@@ -175,8 +176,8 @@ of the most common options to set are:
   <td><code>spark.executor.memory</code></td>
   <td>1g</td>
   <td>
-    Amount of memory to use per executor process, in MiB unless otherwise specified.
-    (e.g. <code>2g</code>, <code>8g</code>).
+    Amount of memory to use per executor process, in the same format as JVM memory strings with
+    a size unit suffix ("k", "m", "g" or "t") (e.g. <code>512m</code>, <code>2g</code>).
   </td>
 </tr>
 <tr>