[VL] Update document, remove the experimental word for spark.gluten.e…

…nabled (apache#8615)

spark.gluten.enabled is stable for long time. It can enable/disable gluten at run time. With it disable, all query plans fallback to vanilla spark. Compare to plugin setting, the config may have a little overhead when generating query plan.

docs/Configuration.md

@@ -21,7 +21,7 @@ You can add these configurations into spark-defaults.conf to enable or disable t
 | spark.sql.join.preferSortMergeJoin                           | When true, prefer sort merge join over shuffled hash join. <br /> Note: Please turn off preferSortMergeJoin.                                                                                                                                                                                                                                                                                                                                                                                                                              | false                                                |
 | spark.plugins                                                | To load Gluten's components by Spark's plug-in loader                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     | org.apache.gluten.GlutenPlugin                       |
 | spark.shuffle.manager                                        | To turn on Gluten Columnar Shuffle Plugin                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 | org.apache.spark.shuffle.sort.ColumnarShuffleManager |
-| spark.gluten.enabled                                         | Enable Gluten, default is true. Just an experimental property. Recommend to enable/disable Gluten through the setting for `spark.plugins`.                                                                                                                                                                                                                                                                                                                                                                                                | true                                                 |
+| spark.gluten.enabled                                         | Enable Gluten at runtime, default is true. It fallbacks to vanilla Spark for all query plans if set to false. Recommend to enable/disable Gluten through the setting for `spark.plugins`.                                                                                                                                                                                                                                                                                                                                                                                                | true                                                 |
 | spark.gluten.memory.isolation                                | (Experimental) Enable isolated memory mode. If true, Gluten controls the maximum off-heap memory can be used by each task to X, X = executor memory / max task slots. It's recommended to set true if Gluten serves concurrent queries within a single session, since not all memory Gluten allocated is guaranteed to be spillable. In the case, the feature should be enabled to avoid OOM. Note when true, setting spark.memory.storageFraction to a lower value is suggested since storage memory is considered non-usable by Gluten. | false                                                |
 | spark.gluten.ras.enabled                                     | Enables RAS (relation algebra selector) during physical planning to generate more efficient query plan. Note, this feature doesn't bring performance profits by default. Try exploring option `spark.gluten.ras.costModel` for advanced usage.                                                                                                                                                                                                                                                                                            | false                                                |
 | spark.gluten.sql.columnar.maxBatchSize                       | Number of rows to be processed in each batch. Default value is 4096.                                                                                                                                                                                                                                                                                                                                                                                                                                                                      | 4096                                                 |