[SPARK-28366][CORE][FOLLOW-UP] Improve the conf IO_WARNING_LARGEFILETHRESHOLD

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

Improve conf `IO_WARNING_LARGEFILETHRESHOLD` (a.k.a `spark.io.warning.largeFileThreshold`):

* reword documentation

* change type from `long` to `bytes`

### Why are the changes needed?

Improvements according to #25134 (comment) & #25134 (comment).

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

No.

### How was this patch tested?

Pass Jenkins.

Closes #26691 from Ngone51/SPARK-28366-followup.

Authored-by: wuyi <ngone_5451@163.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

Author: Ngone51

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

@@ -1325,9 +1325,9 @@ package object config {
   private[spark] val IO_WARNING_LARGEFILETHRESHOLD =
     ConfigBuilder("spark.io.warning.largeFileThreshold")
       .internal()
-      .doc("When spark loading one single large file, if file size exceed this " +
-        "threshold, then log warning with possible reasons.")
-      .longConf
+      .doc("If the size in bytes of a file loaded by Spark exceeds this threshold, " +
+        "a warning is logged with the possible reasons.")
+      .bytesConf(ByteUnit.BYTE)
       .createWithDefault(1024 * 1024 * 1024)
 
   private[spark] val EVENT_LOG_COMPRESSION_CODEC =