AccessibilityInfo: add prefersCrossFadeTransitions to docs (facebook#3279)

Author: gabrieldonadel

docs/accessibilityinfo.md

@@ -210,7 +210,7 @@ Post a string to be announced by the screen reader with modification options. By
 static getRecommendedTimeoutMillis(originalTimeout)
 ```
 
-Gets the timeout in millisecond that the user needs.  
+Gets the timeout in millisecond that the user needs.
 This value is set in "Time to take action (Accessibility timeout)" of "Accessibility" settings.
 
 **Parameters:**
@@ -293,6 +293,16 @@ Query whether a screen reader is currently enabled. Returns a promise which reso
 
 ---
 
+### `prefersCrossFadeTransitions()` <div class="label ios">iOS</div>
+
+```jsx
+static prefersCrossFadeTransitions()
+```
+
+Query whether reduce motion and prefer cross-fade transitions settings are currently enabled. Returns a promise which resolves to a boolean. The result is `true` when prefer cross-fade transitions is enabled and `false` otherwise.
+
+---
+
 ### `removeEventListener()`
 
 ```jsx