docs(lambda): explain NODEJS_LATEST better (#28171)

Some confusion (including myself) surrounding what `NODEJS_LATEST` entails. So I'm clearing it up in this doc update. Basically, `NODEJS_LATEST` points to the latest nodejs region everywhere, and some regions lag behind when an official version is announced. So even though `NODEJS_20` exists, `NODEJS_LATEST` currently points to `NODEJS_18` until node20 has populated to all aws regions, including air-gapped regions.

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packages/aws-cdk-lib/aws-lambda/lib/runtime.ts

@@ -110,7 +110,8 @@ export class Runtime {
   public static readonly NODEJS_20_X = new Runtime('nodejs20.x', RuntimeFamily.NODEJS, { supportsInlineCode: true });
 
   /**
-   * The latest NodeJS version currently available
+   * The latest NodeJS version currently available in ALL regions (not necessarily the latest NodeJS version
+   * available in YOUR region).
    */
   public static readonly NODEJS_LATEST = new Runtime('nodejs18.x', RuntimeFamily.NODEJS, { supportsInlineCode: true, isVariable: true });