[SPARK-39979][SQL][FOLLOW-UP] Make `spark.sql.execution.arrow.useLarg…

…eVarTypes` as an internal configuration

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

This PR is a followup of apache#39572 that hides the `spark.sql.execution.arrow.useLargeVarTypes` configuration as an internal configuration.

### Why are the changes needed?

As described in apache#41569, this feature only works for `mapInArrow`, and other cases cannot be completely supported because of Arrow side limitation, see apache/arrow#35289. Therefore, this PR hides this configuration as an internal one for now.

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

No, this configuration was not released out yet.

### How was this patch tested?

Ran the Scala linter.

Closes apache#41584 from HyukjinKwon/SPARK-39979-followup2.

Authored-by: Hyukjin Kwon <gurwls223@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

sql/catalyst/src/main/scala/org/apache/spark/sql/internal/SQLConf.scala

@@ -2841,8 +2841,9 @@ object SQLConf {
       .doc("When using Apache Arrow, use large variable width vectors for string and binary " +
         "types. Regular string and binary types have a 2GiB limit for a column in a single " +
         "record batch. Large variable types remove this limitation at the cost of higher memory " +
-        "usage per value.")
+        "usage per value. Note that this only works for DataFrame.mapInArrow.")
       .version("3.5.0")
+      .internal()
       .booleanConf
       .createWithDefault(false)