[MINOR][SQL] Refactor the comments in HiveClientImpl.withHiveState

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

This PR refactors three parts of the comments in `HiveClientImpl.withHiveState`

One is about the following comment.
```
// The classloader in clientLoader could be changed after addJar, always use the latest
// classloader.
```
The comment was added in SPARK-10810 (apache#8909) because `IsolatedClientLoader.classLoader` was declared as `var`.
But the field is now `val` and cannot be changed after instanciation.
So, the comment can confuse developers.

One is about the following code and comment.
```
// classloader. We explicitly set the context class loader since "conf.setClassLoader" does
// not do that, and the Hive client libraries may need to load classes defined by the client's
// class loader.
Thread.currentThread().setContextClassLoader(clientLoader.classLoader)
```
It's not trivial why this part is necessary and it's difficult when we can remove this code in the future.
So, I revised the comment by adding the reference of the related JIRA.

And the last one is about the following code and comment.
```
// Replace conf in the thread local Hive with current conf
Hive.get(conf)
```
It's also not trivial why this part is necessary.
I revised the comment by adding the reference of the related discussion.

### Why are the changes needed?

To make code more readable.

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

No.

### How was this patch tested?

It's just a comment refactoring so I add no new test.

Closes apache#32162 from sarutak/refactor-HiveClientImpl.

Authored-by: Kousuke Saruta <sarutak@oss.nttdata.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

Author: sarutak

sql/hive/src/main/scala/org/apache/spark/sql/hive/client/HiveClientImpl.scala

@@ -292,15 +292,17 @@ private[hive] class HiveClientImpl(
   def withHiveState[A](f: => A): A = retryLocked {
     val original = Thread.currentThread().getContextClassLoader
     val originalConfLoader = state.getConf.getClassLoader
-    // The classloader in clientLoader could be changed after addJar, always use the latest
-    // classloader. We explicitly set the context class loader since "conf.setClassLoader" does
+    // We explicitly set the context class loader since "conf.setClassLoader" does
     // not do that, and the Hive client libraries may need to load classes defined by the client's
-    // class loader.
+    // class loader. See SPARK-19804 for more details.
     Thread.currentThread().setContextClassLoader(clientLoader.classLoader)
     state.getConf.setClassLoader(clientLoader.classLoader)
     // Set the thread local metastore client to the client associated with this HiveClientImpl.
     Hive.set(client)
     // Replace conf in the thread local Hive with current conf
+    // with the side-effect of Hive.get(conf) to avoid using out-of-date HiveConf.
+    // See discussion in https://github.com/apache/spark/pull/16826/files#r104606859
+    // for more details.
     Hive.get(conf)
     // setCurrentSessionState will use the classLoader associated
     // with the HiveConf in `state` to override the context class loader of the current