Explain why enabled CommonCommandOptions.enablePlatformSpecificOption…

…s is unused.

RELNOTES: None.
PiperOrigin-RevId: 434392114

src/main/java/com/google/devtools/build/lib/runtime/CommonCommandOptions.java

@@ -52,6 +52,8 @@ public class CommonCommandOptions extends OptionsBase {
       help = "No-op, being removed. See https://github.com/bazelbuild/bazel/issues/13892")
   public Void allIncompatibleChanges;
 
+  // It's by design that this field is unused: this command line option takes effect by reading its
+  // value during options parsing based on its (string) name.
   @Option(
       name = "enable_platform_specific_config",
       defaultValue = "false",