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

**Bug, feature request, or proposal:**
`` 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 :
```

Ce champ est requis


Les adresses mail ne correspondent pas

```

**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 `` tag by a `

` tag (or anything else), it's work.**

The issue names no repository file, test, or documentation entry point; begin with the inline mat-error and parent FormGroup reproduction. Done means adding a clear example of the reported validation case and confirming that the expected error is displayed.

Written by the indexing model from the issue text.