ci: Update comments

Author: lqiu96

.kokoro/nightly/downstream-protobuf-source-compatibility.sh

@@ -28,12 +28,14 @@ for repo in ${REPOS_UNDER_TEST//,/ }; do # Split on comma
   git clone "https://github.com/googleapis/$repo.git" --depth=1
   pushd "$repo"
 
-  # Compile with Java 11 and run the tests with Java 8
+  # Compile with Java 11 and run the tests with Java 8 JVM
   mvn compile -T 1C
 
-  if [ -n "${JAVA8_HOME}"  ]; then
+  # JAVA8_HOME is set by the GH Actions CI
+  if [ -n "${JAVA8_HOME}" ]; then
     surefire_opt="-Djvm=${JAVA8_HOME}/bin/java"
   else
+    # Provide a default value for local executions that don't configure JAVA8_HOME
     surefire_opt="-Djvm=${JAVA_HOME}/bin/java"
   fi