[MINOR][CORE] Expose the alias -c flag of --conf for spark-submit

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

-c is short for --conf, it was introduced since v1.1.0 but hidden from users until now

### Why are the changes needed?

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

no

expose hidden feature

### How was this patch tested?

Nah

Closes #27802 from yaooqinn/conf.

Authored-by: Kent Yao <yaooqinn@hotmail.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

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

@@ -513,7 +513,7 @@ private[deploy] class SparkSubmitArguments(args: Seq[String], env: Map[String, S
         |                              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.
+        |  --conf, -c PROP=VALUE       Arbitrary Spark configuration property.
         |  --properties-file FILE      Path to a file from which to load extra properties. If not
         |                              specified, this will look for conf/spark-defaults.conf.
         |

docs/configuration.md

@@ -95,7 +95,7 @@ Then, you can supply configuration values at runtime:
 
 The Spark shell and [`spark-submit`](submitting-applications.html)
 tool support two ways to load configurations dynamically. The first is command line options,
-such as `--master`, as shown above. `spark-submit` can accept any Spark property using the `--conf`
+such as `--master`, as shown above. `spark-submit` can accept any Spark property using the `--conf/-c`
 flag, but uses special flags for properties that play a part in launching the Spark application.
 Running `./bin/spark-submit --help` will show the entire list of these options.