[SPARK-23238][SQL] Externalize SQLConf configurations exposed in documentation

## What changes were proposed in this pull request?

This PR proposes to expose few internal configurations found in the documentation.

Also it fixes the description for `spark.sql.execution.arrow.enabled`.
It's quite self-explanatory.

## How was this patch tested?

N/A

Author: hyukjinkwon <[email protected]>

Closes #20403 from HyukjinKwon/minor-doc-arrow.

(cherry picked from commit 39d2c6b)
Signed-off-by: hyukjinkwon <[email protected]>

Author: HyukjinKwon

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

@@ -123,14 +123,12 @@ object SQLConf {
       .createWithDefault(10)
 
   val COMPRESS_CACHED = buildConf("spark.sql.inMemoryColumnarStorage.compressed")
-    .internal()
     .doc("When set to true Spark SQL will automatically select a compression codec for each " +
       "column based on statistics of the data.")
     .booleanConf
     .createWithDefault(true)
 
   val COLUMN_BATCH_SIZE = buildConf("spark.sql.inMemoryColumnarStorage.batchSize")
-    .internal()
     .doc("Controls the size of batches for columnar caching.  Larger batch sizes can improve " +
       "memory utilization and compression, but risk OOMs when caching data.")
     .intConf
@@ -1043,11 +1041,11 @@ object SQLConf {
 
   val ARROW_EXECUTION_ENABLE =
     buildConf("spark.sql.execution.arrow.enabled")
-      .internal()
-      .doc("Make use of Apache Arrow for columnar data transfers. Currently available " +
-        "for use with pyspark.sql.DataFrame.toPandas with the following data types: " +
-        "StringType, BinaryType, BooleanType, DoubleType, FloatType, ByteType, IntegerType, " +
-        "LongType, ShortType")
+      .doc("When true, make use of Apache Arrow for columnar data transfers. Currently available " +
+        "for use with pyspark.sql.DataFrame.toPandas, and " +
+        "pyspark.sql.SparkSession.createDataFrame when its input is a Pandas DataFrame. " +
+        "The following data types are unsupported: " +
+        "MapType, ArrayType of TimestampType, and nested StructType.")
       .booleanConf
       .createWithDefault(false)