Removed some limitations. Added Esc key tip.

Incorporating some edits from @paulirish  and Dmitry.

Removed limitations:
"Changes to window.outerWidth/window.outerHeight values based on the DPR are not emulated." --> inaccurate.
"Variations in compositor behavior are not emulated." --> bug fixed.

Added mention of Esc keyboard shortcut to open emulation drawer.