revise documentation for YellowBox API (#583)

Author: sgny

reason-react-native/src/apis/YellowBox.md

@@ -1,13 +1,22 @@
 ---
 id: apis/YellowBox
 title: YellowBox
-wip: true
+officialDoc: https://facebook.github.io/react-native/docs/debugging#warnings
 ---
 
-```reason
-[@bs.module "react-native"] [@bs.scope "YellowBox"]
-external ignoreWarnings: array(string) => unit = "";
+Warning messages are displayed in white on a yellow background; which is known
+as a YellowBox. Warnings may be trigged with the `Js.Console.warn` function included
+in BuckleScript.
+
+## Methods
+
+### `ignoreWarnings`
 
+Allows setting an array of prefixes to be ignored; any warning message which
+begins with a string in that array will not be displayed in the YellowBox.
+
+```reason
+ignoreWarnings: array(string) => unit
 ```
 
 ## Example