PERF-5725 Add comments to the xmls

Author: mbor-dev

ycsb-mongodb/mongodb/pom.xml

@@ -24,6 +24,9 @@
       <artifactId>core</artifactId>
       <version>${project.version}</version>
     </dependency>
+    <!-- mongodb-driver-sync defines snappy-java and zstd-jni dependencies as optional.
+         They need to be repeated here to support compression. See:
+         https://www.mongodb.com/docs/drivers/java/sync/current/fundamentals/connection/network-compression/ -->
     <dependency>
       <groupId>org.xerial.snappy</groupId>
       <artifactId>snappy-java</artifactId>

ycsb-mongodb/pom.xml

@@ -53,6 +53,8 @@
   <!-- Properties Management -->
   <properties>
     <maven.assembly.version>2.2.1</maven.assembly.version>
+    <!-- mongodb, snappy and zstd versions need to be updated together. see:
+         https://www.mongodb.com/docs/drivers/java/sync/current/fundamentals/connection/network-compression/ -->
     <mongodb.version>4.7.0</mongodb.version>
     <snappy.version>1.1.8.4</snappy.version>
     <zstd.version>1.5.0-4</zstd.version>