[CORE][DOCS] Update a help message for --files in spark-submit

## What changes were proposed in this pull request?
This pr is to update a help message for `--files` in spark-submit because it seems users get confused about how to get full paths of the files that one adds via the option.

Author: Takeshi YAMAMURO <linguin.m.s@gmail.com>

Closes #16698 from maropu/SparkFilesDoc.

core/src/main/scala/org/apache/spark/deploy/SparkSubmitArguments.scala

@@ -506,7 +506,8 @@ private[deploy] class SparkSubmitArguments(args: Seq[String], env: Map[String, S
         |  --py-files PY_FILES         Comma-separated list of .zip, .egg, or .py files to place
         |                              on the PYTHONPATH for Python apps.
         |  --files FILES               Comma-separated list of files to be placed in the working
-        |                              directory of each executor.
+        |                              directory of each executor. File paths of these files
+        |                              in executors can be accessed via SparkFiles.get(fileName).
         |
         |  --conf PROP=VALUE           Arbitrary Spark configuration property.
         |  --properties-file FILE      Path to a file from which to load extra properties. If not