[SPARK-31055][DOCS] Update config docs for shuffle local host reads to have dep on external shuffle service

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

with SPARK-27651 we now support host local reads for shuffle, but only when external shuffle service is enabled. Update the config docs to state that.

### Why are the changes needed?

clarify dependency

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

no

### How was this patch tested?

n/a

Closes #27812 from tgravescs/SPARK-27651-follow.

Authored-by: Thomas Graves <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: tgravescs

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

@@ -1216,7 +1216,8 @@ package object config {
 
   private[spark] val SHUFFLE_HOST_LOCAL_DISK_READING_ENABLED =
     ConfigBuilder("spark.shuffle.readHostLocalDisk")
-      .doc(s"If enabled (and `${SHUFFLE_USE_OLD_FETCH_PROTOCOL.key}` is disabled), shuffle " +
+      .doc(s"If enabled (and `${SHUFFLE_USE_OLD_FETCH_PROTOCOL.key}` is disabled and external " +
+        s"shuffle `${SHUFFLE_SERVICE_ENABLED.key}` is enabled), shuffle " +
         "blocks requested from those block managers which are running on the same host are read " +
         "from the disk directly instead of being fetched as remote blocks over the network.")
       .booleanConf