[SPARK-34235][SS] Make spark.sql.hive as a private package

### What changes were proposed in this pull request?
Follow the comment https://github.com/apache/spark/pull/31271#discussion_r562598983:

- Remove the API tag `Unstable` for `HiveSessionStateBuilder`
- Add document for spark.sql.hive package to emphasize it's a private package

### Why are the changes needed?
Follow the rule for a private package.

### Does this PR introduce _any_ user-facing change?
No.

### How was this patch tested?
Doc change only.

Closes #31321 from xuanyuanking/SPARK-34185-follow.

Authored-by: Yuanjian Li <yuanjian.li@databricks.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

project/SparkBuild.scala

@@ -910,7 +910,7 @@ object Unidoc {
       .map(_.filterNot(_.getCanonicalPath.contains("org/apache/spark/sql/catalyst")))
       .map(_.filterNot(_.getCanonicalPath.contains("org/apache/spark/sql/execution")))
       .map(_.filterNot(_.getCanonicalPath.contains("org/apache/spark/sql/internal")))
-      .map(_.filterNot(_.getCanonicalPath.contains("org/apache/spark/sql/hive/test")))
+      .map(_.filterNot(_.getCanonicalPath.contains("org/apache/spark/sql/hive")))
       .map(_.filterNot(_.getCanonicalPath.contains("org/apache/spark/sql/catalog/v2/utils")))
       .map(_.filterNot(_.getCanonicalPath.contains("org/apache/hive")))
       .map(_.filterNot(_.getCanonicalPath.contains("org/apache/spark/sql/v2/avro")))

sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveSessionStateBuilder.scala

@@ -19,7 +19,6 @@ package org.apache.spark.sql.hive
 
 import java.net.URI
 
-import org.apache.spark.annotation.Unstable
 import org.apache.spark.sql._
 import org.apache.spark.sql.catalyst.analysis.{Analyzer, ResolveSessionCatalog}
 import org.apache.spark.sql.catalyst.catalog.ExternalCatalogWithListener
@@ -38,7 +37,6 @@ import org.apache.spark.sql.internal.{BaseSessionStateBuilder, SessionResourceLo
 /**
  * Builder that produces a Hive-aware `SessionState`.
  */
-@Unstable
 class HiveSessionStateBuilder(
     session: SparkSession,
     parentState: Option[SessionState],

sql/hive/src/main/scala/org/apache/spark/sql/hive/package.scala

@@ -23,5 +23,8 @@ package org.apache.spark.sql
  *  - Using HiveQL to express queries.
  *  - Reading metadata from the Hive Metastore using HiveSerDes.
  *  - Hive UDFs, UDAs, UDTs
+ *
+ * Note that this is a private package. All classes in this package are considered an internal API
+ * to Spark and are subject to change between minor releases.
  */
 package object hive