[input] Add example of using mat-error with parent formgroup validation

Bug, feature request, or proposal: <mat-error> doesn't show when I use an email maching validator for emails inputs.

What is the expected behavior? To display the error.

What is the current behavior? No error is displayed

What are the steps to reproduce?

Set a custom validator for check if two emails inputs are equals, like this :

private matchEmail(AC: AbstractControl) {
    return AC.get('mail').value === AC.get('mailconfirm').value ? null : { mailmismatch: true };
}

this.administratifForm = this.fb.group({
        (...),
        mail: this.fb.control('', [Validators.required, Validators.email]),
        mailconfirm: this.fb.control('', [Validators.required]),
        (...),
    }, {
    validator: this.matchEmail,
    },
);

The template :

<mat-form-field>
    <input matInput placeholder="Vérification d'email" formControlName="mailconfirm">
    <mat-error *ngIf="administratifForm.get('mailconfirm').hasError('required')">
        Ce champ est requis
    </mat-error>
    <mat-error *ngIf="administratifForm.hasError('mailmismatch')">
        Les adresses mail ne correspondent pas
    </mat-error>
</mat-form-field>

Which versions of Angular, Material, OS, TypeScript, browsers are affected? Angular 5.0.2, Material 5.0.0-rc0, MacOS Sierra, Firefox

Additional Information If i replace the <mat-error> tag by a <p> tag (or anything else), it's work.