Document the async versions of getZoomFactor and getZoomLevel

docs/api/web-contents.md

@@ -623,6 +623,10 @@ Returns whether this page has been muted.
 Changes the zoom factor to the specified factor. Zoom factor is
 zoom percent divided by 100, so 300% = 3.0.
 
+#### `contents.getZoomFactor(callback)`
+
+`callback` is called with the current zoom factor
+
 #### `contents.setZoomLevel(level)`
 
 * `level` Number - Zoom level
@@ -631,6 +635,10 @@ Changes the zoom level to the specified level. The original size is 0 and each
 increment above or below represents zooming 20% larger or smaller to default
 limits of 300% and 50% of original size, respectively.
 
+#### `contents.getZoomLevel(callback)`
+
+`callback` is called with the current zoom level
+
 #### `contents.setZoomLevelLimits(minimumLevel, maximumLevel)`
 
 * `minimumLevel` Number