[SPARK-42861][SQL] Use private[sql] instead of protected[sql] to avoid generating API doc

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

This is the only issue I found during SQL module API auditing via apache/spark-website@6159860 . Somehow `protected[sql]` also generates API doc which is unexpected. `private[sql]` solves the problem and I generated doc locally to verify it.

Another API issue has been fixed by apache#40499

### Why are the changes needed?

fix api doc

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

no

### How was this patch tested?

N/A

Closes apache#40541 from cloud-fan/auditing.

Authored-by: Wenchen Fan <[email protected]>
Signed-off-by: Kent Yao <[email protected]>

Author: cloud-fan

sql/core/src/main/scala/org/apache/spark/sql/RuntimeConfig.scala

@@ -64,7 +64,7 @@ class RuntimeConfig private[sql](sqlConf: SQLConf = new SQLConf) {
   /**
    * Sets the given Spark runtime configuration property.
    */
-  protected[sql] def set[T](entry: ConfigEntry[T], value: T): Unit = {
+  private[sql] def set[T](entry: ConfigEntry[T], value: T): Unit = {
     requireNonStaticConf(entry.key)
     sqlConf.setConf(entry, value)
   }
@@ -94,18 +94,18 @@ class RuntimeConfig private[sql](sqlConf: SQLConf = new SQLConf) {
    * Returns the value of Spark runtime configuration property for the given key.
    */
   @throws[NoSuchElementException]("if the key is not set")
-  protected[sql] def get[T](entry: ConfigEntry[T]): T = {
+  private[sql] def get[T](entry: ConfigEntry[T]): T = {
     sqlConf.getConf(entry)
   }
 
-  protected[sql] def get[T](entry: OptionalConfigEntry[T]): Option[T] = {
+  private[sql] def get[T](entry: OptionalConfigEntry[T]): Option[T] = {
     sqlConf.getConf(entry)
   }
 
   /**
    * Returns the value of Spark runtime configuration property for the given key.
    */
-  protected[sql] def get[T](entry: ConfigEntry[T], default: T): T = {
+  private[sql] def get[T](entry: ConfigEntry[T], default: T): T = {
     sqlConf.getConf(entry, default)
   }
 
@@ -153,7 +153,7 @@ class RuntimeConfig private[sql](sqlConf: SQLConf = new SQLConf) {
   /**
    * Returns whether a particular key is set.
    */
-  protected[sql] def contains(key: String): Boolean = {
+  private[sql] def contains(key: String): Boolean = {
     sqlConf.contains(key)
   }