add comment regarding scheduling license with future issue date

Original commit: elastic/x-pack-elasticsearch@7d4094da1e

elasticsearch/x-pack/license-plugin/src/main/java/org/elasticsearch/license/plugin/core/LicenseSchedule.java

@@ -31,6 +31,11 @@ public class LicenseSchedule implements SchedulerEngine.Schedule {
                 break;
             case DISABLED:
                 if (license.issueDate() > time) {
+                    // when we encounter a license with a future issue date
+                    // which can happen with autogenerated license,
+                    // we want to schedule a notification on the license issue date
+                    // so the license is notificed once it is valid
+                    // see https://github.com/elastic/x-plugins/issues/983
                     nextScheduledTime = license.issueDate();
                 }
                 break;