docs(cognito): update user pool example with case insensitive sign in (#22462)

Making usernames case insensitive in basic example because that is preferred in most situations.


----

### All Submissions:

* [Y] Have you followed the guidelines in our [Contributing guide?](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md)

### Adding new Unconventional Dependencies:

* [N] 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

* [N] 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`)?

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: timwhunt

packages/@aws-cdk/aws-cognito/README.md

@@ -66,9 +66,15 @@ the `userPoolName` to give your own identifier to the user pool. If not, CloudFo
 ```ts
 new cognito.UserPool(this, 'myuserpool', {
   userPoolName: 'myawesomeapp-userpool',
+  signInCaseSensitive: false, // case insensitive is preferred in most situations
 });
 ```
 
+By default, usernames and email addresses in user pools are case sensitive, which means `[email protected]` and `[email protected]`
+are considered different. In most situations it is prefered to have usernames and email addresses be case insensitive so that
+capitalization differences are ignored. As shown above, you can make a user pool case insensitive by setting `signInCaseSensitive`
+to `false`. The case sensitivity cannot be changed once a user pool is created.
+
 The default set up for the user pool is configured such that only administrators will be allowed
 to create users. Features such as Multi-factor authentication (MFAs) and Lambda Triggers are not
 configured by default.