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
2. Notice how the checkboxes aren't themed
3. Open https://stackblitz.com/edit/angular-sbrzfb?file=app/input-overview-example.ts
4. 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