Explain why enabled CommonCommandOptions.enablePlatformSpecificOptions is unused.

RELNOTES: None.
PiperOrigin-RevId: 434392114
diff --git a/src/main/java/com/google/devtools/build/lib/runtime/CommonCommandOptions.java b/src/main/java/com/google/devtools/build/lib/runtime/CommonCommandOptions.java
index b481b8c..006ba66 100644
--- a/src/main/java/com/google/devtools/build/lib/runtime/CommonCommandOptions.java
+++ b/src/main/java/com/google/devtools/build/lib/runtime/CommonCommandOptions.java
@@ -52,6 +52,8 @@
       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",