Document fullscreen/ESC behaviour.

And the fact that the user can leave fullscreen by pressing ESC but this
can be prevented by the developer by cancelling the event and requesting
a special permission.

BUG=320487

Review URL: https://codereview.chromium.org/116083008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243087 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243087