docs(lambda): clarify x-ray sample rate (#23122)

The current description is not accurate.

----

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Author: mrgrain

packages/@aws-cdk/aws-lambda/lib/function.ts

@@ -33,7 +33,7 @@ import { addAlias } from './util';
 export enum Tracing {
   /**
    * Lambda will respect any tracing header it receives from an upstream service.
-   * If no tracing header is received, Lambda will call X-Ray for a tracing decision.
+   * If no tracing header is received, Lambda will sample the request based on a fixed rate. Please see the [Using AWS Lambda with AWS X-Ray](https://docs.aws.amazon.com/lambda/latest/dg/services-xray.html) documentation for details on this sampling behavior.
    */
   ACTIVE = 'Active',
   /**