[SPARK-16829][SPARKR] sparkR sc.setLogLevel doesn't work

(Please fill in changes proposed in this fix)

./bin/sparkR
Launching java with spark-submit command /Users/mwang/spark_ws_0904/bin/spark-submit "sparkr-shell" /var/folders/s_/83b0sgvj2kl2kwq4stvft_pm0000gn/T//RtmpQxJGiZ/backend_porte9474603ed1e
Using Spark's default log4j profile: org/apache/spark/log4j-defaults.properties
Setting default log level to "WARN".
To adjust logging level use sc.setLogLevel(newLevel).

> sc.setLogLevel("INFO")
Error: could not find function "sc.setLogLevel"

sc.setLogLevel doesn't exist.

R has a function setLogLevel.

I rename the setLogLevel function to sc.setLogLevel.

(Please explain how this patch was tested. E.g. unit tests, integration tests, manual tests)
Change unit test. Run unit tests.
Manually tested it in sparkR shell.

Author: wm624@hotmail.com <wm624@hotmail.com>

Closes #14433 from wangmiao1981/sc.

core/src/main/scala/org/apache/spark/internal/Logging.scala

@@ -135,7 +135,8 @@ private[spark] trait Logging {
         val replLevel = Option(replLogger.getLevel()).getOrElse(Level.WARN)
         if (replLevel != rootLogger.getEffectiveLevel()) {
           System.err.printf("Setting default log level to \"%s\".\n", replLevel)
-          System.err.println("To adjust logging level use sc.setLogLevel(newLevel).")
+          System.err.println("To adjust logging level use sc.setLogLevel(newLevel). " +
+            "For SparkR, use setLogLevel(newLevel).")
           rootLogger.setLevel(replLevel)
         }
       }