[Dialog] Add accessibility section in the documentation

[chedvac]

• This is not a v0.x issue.
• I have searched the issues of this repository and believe that this is not a duplicate.

Expected Behavior 🤔

According to wai-aria guidelines, when dialog is fired - Initial focus should set on the first focusable element.

see on: https://www.w3.org/TR/wai-aria-practices/examples/dialog-modal/dialog.html

Current Behavior 😯

The focus is trapped on the dialog, but not set on the first element.
Need to press 'Tab' to focus to first element.

Your Environment 🌎

Tech	Version
Material-UI	v3.5.1
React	v16.6.3
Browser	chrome 70.0.3538.102

[oliviertassinari]

According to wai-aria guidelines, when dialog is fired - Initial focus should set on the first focusable element.

@chedvac The specification mentions 4 different dialog focus strategy. Initial focus should set on the first focusable element is just one of them. I don't think that it's something we should enforce.

Now, I think that we miss an accessibility documentation section for the Dialog. I think that we should add one!