Add comment clarifying purpose of $NoTargeting

Author: rmi22186

src/constants.ts

@@ -225,6 +225,9 @@ export default Constants;
 export const ONE_DAY_IN_SECONDS = 60 * 60 * 24;
 export const MILLIS_IN_ONE_SEC = 1000;
 
+// User attribute set when the end user opts out of ad targeting via
+// launcherOptions.noTargeting. Server-side systems use this attribute
+// to exclude the user from retargeting audiences.
 export const NO_TARGETING_ATTRIBUTE = '$NoTargeting';
 
 // https://developer.mozilla.org/en-US/docs/Web/HTTP/Status