[SPARK-28869][DOCS][FOLLOWUP] Add direct relationship between configs for rolling event log

### What changes were proposed in this pull request?

This patch addresses the post-hoc review comment linked here - #25670 (comment)

### Why are the changes needed?

We would like to explicitly document the direct relationship before we finish up structuring of configurations.

### Does this PR introduce any user-facing change?

No.

### How was this patch tested?

N/A

Closes #27576 from HeartSaVioR/SPARK-28869-FOLLOWUP-doc.

Authored-by: Jungtaek Lim (HeartSaVioR) <kabhwan.opensource@gmail.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
(cherry picked from commit 446b2d2)
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

Author: HeartSaVioR

core/src/main/scala/org/apache/spark/internal/config/package.scala

@@ -191,14 +191,15 @@ package object config {
 
   private[spark] val EVENT_LOG_ENABLE_ROLLING =
     ConfigBuilder("spark.eventLog.rolling.enabled")
-      .doc("Whether rolling over event log files is enabled.  If set to true, it cuts down " +
+      .doc("Whether rolling over event log files is enabled. If set to true, it cuts down " +
         "each event log file to the configured size.")
       .booleanConf
       .createWithDefault(false)
 
   private[spark] val EVENT_LOG_ROLLING_MAX_FILE_SIZE =
     ConfigBuilder("spark.eventLog.rolling.maxFileSize")
-      .doc("The max size of event log file to be rolled over.")
+      .doc(s"When ${EVENT_LOG_ENABLE_ROLLING.key}=true, specifies the max size of event log file" +
+        " to be rolled over.")
       .bytesConf(ByteUnit.BYTE)
       .checkValue(_ >= ByteUnit.MiB.toBytes(10), "Max file size of event log should be " +
         "configured to be at least 10 MiB.")

docs/configuration.md

@@ -1033,7 +1033,7 @@ Apart from these, the following properties are also available, and may be useful
   <td><code>spark.eventLog.rolling.maxFileSize</code></td>
   <td>128m</td>
   <td>
-    The max size of event log file before it's rolled over.
+    When <code>spark.eventLog.rolling.enabled=true</code>, specifies the max size of event log file before it's rolled over.
   </td>
 </tr>
 <tr>