[ErrorProne] Update comment in BeamModulePlugin.groovy (#38281)

Intended suppressions with justifications

Author: stankiewicz

buildSrc/src/main/groovy/org/apache/beam/gradle/BeamModulePlugin.groovy

@@ -1548,6 +1548,7 @@ class BeamModulePlugin implements Plugin<Project> {
             "MockNotUsedInProduction",
             "NullableWildcard",
             "SuperCallToObjectMethod",
+            // Intended suppressions with justifications
             // for encoding efficiency and backward compatibility
             "EnumOrdinal",
             // widely used in non-public methods