[SPARK-33208][SQL] Update the document of SparkSession#sql

Change-Id: I82db1f9e8f667573aa3a03e05152cbed0ea7686b

### What changes were proposed in this pull request?
Update the document of SparkSession#sql, mention that this API eagerly runs DDL/DML commands, but not for SELECT queries.

### Why are the changes needed?
To clarify the behavior of SparkSession#sql.

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

### How was this patch tested?
No needed.

Closes #30168 from waitinfuture/master.

Authored-by: zky.zhoukeyong <zky.zhoukeyong@alibaba-inc.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

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

@@ -641,8 +641,8 @@ class SQLContext private[sql](val sparkSession: SparkSession)
   }
 
   /**
-   * Executes a SQL query using Spark, returning the result as a `DataFrame`. The dialect that is
-   * used for SQL parsing can be configured with 'spark.sql.dialect'.
+   * Executes a SQL query using Spark, returning the result as a `DataFrame`.
+   * This API eagerly runs DDL/DML commands, but not for SELECT queries.
    *
    * @group basic
    * @since 1.3.0

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

@@ -600,7 +600,7 @@ class SparkSession private(
 
   /**
    * Executes a SQL query using Spark, returning the result as a `DataFrame`.
-   * The dialect that is used for SQL parsing can be configured with 'spark.sql.dialect'.
+   * This API eagerly runs DDL/DML commands, but not for SELECT queries.
    *
    * @since 2.0.0
    */