docs: add note about the month being zero-based in the Date constructor (#37770)

Because the month is zero based, it may confuse some users that '3'
is in fact 'April'. This comment should clear that up.

PR Close #37770

aio/content/examples/pipes/src/app/app.component.ts

@@ -6,5 +6,5 @@ import { Component } from '@angular/core';
   templateUrl: './app.component.html'
 })
 export class AppComponent {
-  birthday = new Date(1988, 3, 15); // April 15, 1988
+  birthday = new Date(1988, 3, 15); // April 15, 1988 -- since month parameter is zero-based
 }

aio/content/examples/pipes/src/app/hero-birthday1.component.ts

@@ -8,5 +8,5 @@ import { Component } from '@angular/core';
   // #enddocregion hero-birthday-template
 })
 export class HeroBirthdayComponent {
-  birthday = new Date(1988, 3, 15); // April 15, 1988
+  birthday = new Date(1988, 3, 15); // April 15, 1988 -- since month parameter is zero-based
 }

aio/content/examples/pipes/src/app/hero-birthday2.component.ts

@@ -12,7 +12,7 @@ import { Component } from '@angular/core';
 })
 // #docregion class
 export class HeroBirthday2Component {
-  birthday = new Date(1988, 3, 15); // April 15, 1988
+  birthday = new Date(1988, 3, 15); // April 15, 1988 -- since month parameter is zero-based
   toggle = true; // start with true == shortDate
 
   get format()   { return this.toggle ? 'shortDate' : 'fullDate'; }