[SPARK-19724][SQL] allowCreatingManagedTableUsingNonemptyLocation should have legacy prefix

docs/sql-programming-guide.md

@@ -1951,7 +1951,7 @@ working with timestamps in `pandas_udf`s to get the best performance, see
   - Since Spark 2.4, expression IDs in UDF arguments do not appear in column names. For example, an column name in Spark 2.4 is not `UDF:f(col0 AS colA#28)` but ``UDF:f(col0 AS `colA`)``.
   - Since Spark 2.4, writing a dataframe with an empty or nested empty schema using any file formats (parquet, orc, json, text, csv etc.) is not allowed. An exception is thrown when attempting to write dataframes with empty schema.
   - Since Spark 2.4, Spark compares a DATE type with a TIMESTAMP type after promotes both sides to TIMESTAMP. To set `false` to `spark.sql.legacy.compareDateTimestampInTimestamp` restores the previous behavior. This option will be removed in Spark 3.0.
-  - Since Spark 2.4, creating a managed table with nonempty location is not allowed. An exception is thrown when attempting to create a managed table with nonempty location. To set `true` to `spark.sql.allowCreatingManagedTableUsingNonemptyLocation` restores the previous behavior. This option will be removed in Spark 3.0.
+  - Since Spark 2.4, creating a managed table with nonempty location is not allowed. An exception is thrown when attempting to create a managed table with nonempty location. To set `true` to `spark.sql.legacy.allowCreatingManagedTableUsingNonemptyLocation` restores the previous behavior. This option will be removed in Spark 3.0.
   - Since Spark 2.4, renaming a managed table to existing location is not allowed. An exception is thrown when attempting to rename a managed table to existing location.
   - Since Spark 2.4, the type coercion rules can automatically promote the argument types of the variadic SQL functions (e.g., IN/COALESCE) to the widest common type, no matter how the input arguments order. In prior Spark versions, the promotion could fail in some specific orders (e.g., TimestampType, IntegerType and StringType) and throw an exception.
   - Since Spark 2.4, Spark has enabled non-cascading SQL cache invalidation in addition to the traditional cache invalidation mechanism. The non-cascading cache invalidation mechanism allows users to remove a cache without impacting its dependent caches. This new cache invalidation mechanism is used in scenarios where the data of the cache to be removed is still valid, e.g., calling unpersist() on a Dataset, or dropping a temporary view. This allows users to free up memory and keep the desired caches valid at the same time.

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

@@ -1358,7 +1358,7 @@ object SQLConf {
     .createWithDefault(false)
 
   val ALLOW_CREATING_MANAGED_TABLE_USING_NONEMPTY_LOCATION =
-    buildConf("spark.sql.allowCreatingManagedTableUsingNonemptyLocation")
+    buildConf("spark.sql.legacy.allowCreatingManagedTableUsingNonemptyLocation")
     .internal()
     .doc("When this option is set to true, creating managed tables with nonempty location " +
       "is allowed. Otherwise, an analysis exception is thrown. ")