Add extra comments regarding the flex wrapper.

Author: crisbeto

src/lib/core/overlay/_overlay.scss

@@ -25,7 +25,10 @@
     z-index: $md-z-index-overlay-container;
   }
 
-  // Wrapper element for the panes that use the GlobalPositionStrategy.
+  // We use an extra wrapper element in order to use make the overlay itself a flex item.
+  // This makes centering the overlay easy without running into the subpixel rendering
+  // problems tied to using `transform` and without interfering with the other position
+  // strategies.
   .md-global-overlay-wrapper {
     display: flex;
     position: absolute;

src/lib/core/overlay/position/global-position-strategy.ts

@@ -15,6 +15,8 @@ export class GlobalPositionStrategy implements PositionStrategy {
   private _rightOffset: string = '';
   private _alignItems: string = '';
   private _justifyContent: string = '';
+
+  /* A lazily-created wrapper for the overlay element that is used as a flex container.  */
   private _wrapper: HTMLElement;
 
   /** Sets the top position of the overlay. Clears any previously set vertical position. */