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

Start by reviewing the two linked StackBlitz examples and the Angular documentation for ViewEncapsulation.ShadowDom. Update the relevant documentation to state the support status and explain the observed theming and styling limitation; done when developers can find that guidance.

Written by the indexing model from the issue text.