Updated documentation for hairlineWidth

Summary:
Not seeing a hairline thick line is a common source of confusion with a simple cause. Let's warn about it.
Closes https://github.com/facebook/react-native/pull/9589

Differential Revision: D3770879

Pulled By: mkonicek

fbshipit-source-id: f1637e17832982fa8fb9b7f485056c452b38b3db

Libraries/StyleSheet/StyleSheet.js

@@ -95,6 +95,8 @@ module.exports = {
    * on the underlying platform. However, you should not rely on it being a
    * constant size, because on different platforms and screen densities its
    * value may be calculated differently.
+   * 
+   * A line with hairline width may not be visible if your simulator is downscaled.
    */
   hairlineWidth,