docs: add note about stored context when looking up VPC AZs (#25446)

Adding this note to be explicitly clear

Closes #16250 

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

Author: peterwoodworth

packages/aws-cdk-lib/aws-ec2/README.md

@@ -75,7 +75,13 @@ and *account* of the Stack containing the VPC. If the [region and account are
 specified](https://docs.aws.amazon.com/cdk/latest/guide/environments.html) on
 the Stack, the CLI will [look up the existing Availability
 Zones](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-regions-availability-zones.html#using-regions-availability-zones-describe)
-and get an accurate count. If region and account are not specified, the stack
+and get an accurate count. The result of this operation will be written to a file
+called `cdk.context.json`. You must commit this file to source control so
+that the lookup values are available in non-privileged environments such
+as CI build steps, and to ensure your template builds are repeatable.
+
+
+If region and account are not specified, the stack
 could be deployed anywhere and it will have to make a safe choice, limiting
 itself to 2 Availability Zones.