Add to PixelRation documentation information about iPhone SE and iPho… (

facebook#20695)

Summary:
Release Notes:
--------------

[DOCS] [BUGFIX][PixelRatio.js] - Add information about iPhone SE and iPhone X
Pull Request resolved: facebook#20695

Differential Revision: D9408433

Pulled By: hramos

fbshipit-source-id: 917390e30801ce353fd999b375c788bd38568a05

Libraries/Utilities/PixelRatio.js

@@ -68,9 +68,15 @@ class PixelRatio {
    *     - iPhone 4, 4S
    *     - iPhone 5, 5c, 5s
    *     - iPhone 6
+   *     - iPhone 7
+   *     - iPhone 8
+   *     - iPhone SE
    *     - xhdpi Android devices (320 dpi)
    *   - PixelRatio.get() === 3
-   *     - iPhone 6 plus
+   *     - iPhone 6 Plus
+   *     - iPhone 7 Plus
+   *     - iPhone 8 Plus
+   *     - iPhone X
    *     - xxhdpi Android devices (480 dpi)
    *   - PixelRatio.get() === 3.5
    *     - Nexus 6