docs: clarify support for ViewEncapsulation.ShadowDom

#### Proposal

#### What is the expected behavior?
If `ViewEncapsulation.ShadowDom` is not supported, this should be called out in the docs to save developers time and frustration.

#### What is the current behavior?
`ViewEncapsulation.ShadowDom` is not supported and is not documented.

#### What are the steps to reproduce?
1. Open https://stackblitz.com/edit/angular-i5jw3k?file=app/table-selection-example.ts
1. Notice how the checkboxes aren't themed
1. Open https://stackblitz.com/edit/angular-sbrzfb?file=app/input-overview-example.ts
1. Notice how the inputs are not themed or styled properly

#### What is the use-case or motivation for changing an existing behavior?
To save developers time and frustration.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Angular: 7.0.0
Angular Material: 7.0.2
TypeScript: ~3.1.1
Browsers: Tested on Chrome, Firefox, and Safari latest stable versions

#### Is there anything else we should know?
Nope

Inizia esaminando i due esempi StackBlitz collegati e la documentazione Angular relativa a ViewEncapsulation.ShadowDom. Aggiorna la documentazione pertinente per indicare lo stato di supporto e spiegare la limitazione osservata relativa ai temi e agli stili; il lavoro è completato quando gli sviluppatori possono trovare queste indicazioni.

Scritto dal modello di indicizzazione a partire dal testo della issue.