[FLINK-37440] Fix the bug that parallelism.default do not always adopts 1 as the default value.

Author: beliefer

flink-clients/src/test/java/org/apache/flink/client/program/ClientTest.java

@@ -495,7 +495,6 @@ public boolean isCompatibleWith(@Nonnull Configuration configuration) {
                 @Override
                 public PipelineExecutor getExecutor(@Nonnull Configuration configuration) {
                     return (pipeline, config, classLoader) -> {
-                        final int parallelism = config.get(CoreOptions.DEFAULT_PARALLELISM);
                         final JobGraph jobGraph = streamGraph.getJobGraph();
                         // The job graphs from different cases are generated from the same stream
                         // graph, resulting in the same job ID, which can lead to exceptions.

flink-core/src/main/java/org/apache/flink/configuration/CoreOptions.java

@@ -459,7 +459,8 @@ public static String[] mergeListsToArray(List<String> base, List<String> append)
             ConfigOptions.key("parallelism.default")
                     .intType()
                     .defaultValue(1)
-                    .withDescription("Default parallelism for jobs.");
+                    .withDescription(
+                            "Default parallelism for jobs. There are two special case. The first case is when creating a StreamExecutioneEnvironment, if the value of parallelism.default is not explicitly specified, the number of processors available to the Java virtual machine will be used as the default parallelism. The second case is when creating mini cluster, the number of slots will be used as the default parallelism");
 
     // ------------------------------------------------------------------------
     //  file systems