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

The current description is not accurate.

----

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### Adding new Unconventional Dependencies:

* [ ] This PR adds new unconventional dependencies following the process described [here](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md/#adding-new-unconventional-dependencies)

### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/main/INTEGRATION_TESTS.md)?
	* [ ] Did you use `yarn integ` to deploy the infrastructure and generate the snapshot (i.e. `yarn integ` without `--dry-run`)?

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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',
   /**