refactor(docs-infra): fix docs examples for tslint rules related to underscores in variable names (#38143)

This commit updates the docs examples to be compatible with the
`variable-name` tslint rule without requiring the
`allow-leading-underscore` and `allow-trailing-underscore` options.

This is in preparation of updating the docs examples `tslint.json` to
match the one generated for new Angular CLI apps in a future commit.

PR Close #38143

aio/content/examples/elements/src/app/popup.component.ts

@@ -1,3 +1,5 @@
+// tslint:disable: variable-name
+// #docregion
 import { Component, EventEmitter, Input, Output } from '@angular/core';
 import { animate, state, style, transition, trigger } from '@angular/animations';
 
@@ -42,12 +44,12 @@ export class PopupComponent {
   state: 'opened' | 'closed' = 'closed';
 
   @Input()
+  get message(): string { return this._message; }
   set message(message: string) {
     this._message = message;
     this.state = 'opened';
   }
-  get message(): string { return this._message; }
-  _message: string;
+  private _message: string;
 
   @Output()
   closed = new EventEmitter();