docs(cloudwatch): misleading documentation for metric unit NONE (#20249)

Fixes #20112. Changes documentation for metric unit `NONE` in CloudWatch to avoid any confusions between `NONE` and `No unit`.

----

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Author: daschaa

packages/@aws-cdk/aws-cloudwatch/lib/metric-types.ts

@@ -216,7 +216,7 @@ export enum Unit {
   COUNT_PER_SECOND = 'Count/Second',
 
   /**
-   * No unit
+   * None
    */
   NONE = 'None'
 }