add the comment to unit test

Signed-off-by: kkewwei <kkewwei@163.com>

server/src/internalClusterTest/java/org/opensearch/search/query/SimpleQueryStringIT.java

@@ -112,6 +112,9 @@ public static Collection<Object[]> parameters() {
 
     @BeforeClass
     public static void createRandomClusterSetting() {
+        // Lower bound can't be small(such as 60), simpleQueryStringQuery("foo Bar 19 127.0.0.1") in testDocWithAllTypes
+        // will create many clauses of BooleanClause, In that way, it will throw too_many_nested_clauses exception.
+        // So we need to set a higher bound(such as 80) to avoid failures.
         CLUSTER_MAX_CLAUSE_COUNT = randomIntBetween(80, 100);
     }