docs(ec2): clarify error message for vpc sizing error (#27550)

Subnet size must be minimum /28. This clarifies the error message.

Closes #27479.

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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: msambol

packages/aws-cdk-lib/aws-ec2/lib/network-util.ts

@@ -4,7 +4,7 @@
  */
 export class InvalidCidrRangeError extends Error {
   constructor(cidr: string) {
-    super(cidr + ' is not a valid VPC CIDR range (must be between /16 and /28)');
+    super(cidr + ' is not a valid VPC CIDR range. VPCs must be between /16 and /28 and the minimum subnet size is /28.');
     // The following line is required for type checking of custom errors, and must be called right after super()
     // https://stackoverflow.com/questions/31626231/custom-error-class-in-typescript
     Object.setPrototypeOf(this, InvalidCidrRangeError.prototype);