Add comment that npm transitive dependencies are not needed

Author: ahornace

opengrok-web/pom.xml

@@ -124,6 +124,7 @@ Portions Copyright (c) 2018, 2020, Chris Fraire <[email protected]>.
             <groupId>org.webjars.npm</groupId>
             <artifactId>showdown</artifactId>
             <version>1.9.1</version>
+            <!-- dependencies not required -->
             <exclusions>
                 <exclusion>
                     <groupId>*</groupId>
@@ -135,6 +136,7 @@ Portions Copyright (c) 2018, 2020, Chris Fraire <[email protected]>.
             <groupId>org.webjars.npm</groupId>
             <artifactId>tablesorter</artifactId>
             <version>2.31.3</version>
+            <!-- dependencies not required -->
             <exclusions>
                 <exclusion>
                     <groupId>*</groupId>