Updated docs for beginFrameSubscription

docs/api/web-contents.md

@@ -917,12 +917,14 @@ For the `mouseWheel` event, the `event` object also have following properties:
 * `hasPreciseScrollingDeltas` Boolean
 * `canScroll` Boolean
 
-### `webContents.beginFrameSubscription(callback)`
+### `webContents.beginFrameSubscription(callback, onlyDamaged)`
 
 * `callback` Function
+* `onlyDamaged` Boolean
 
 Begin subscribing for presentation events and captured frames, the `callback`
-will be called with `callback(frameBuffer)` when there is a presentation event.
+will be called with `callback(frameBuffer, damagedRect)` when there is a
+presentation event.
 
 The `frameBuffer` is a `Buffer` that contains raw pixel data. On most machines,
 the pixel data is effectively stored in 32bit BGRA format, but the actual
@@ -930,6 +932,11 @@ representation depends on the endianness of the processor (most modern
 processors are little-endian, on machines with big-endian processors the data
 is in 32bit ARGB format).
 
+The `damagedRect` is an object with `x, y, width, height` properties that
+describes which part of the page was repainted. If `onlyDamaged` is set to
+`true`, `frameBuffer` will only contain the repainted area. `onlyDamaged`
+defaults to `false`.
+
 ### `webContents.endFrameSubscription()`
 
 End subscribing for frame presentation events.