Document reasoning behind hairlineWidth value

Close #1488

packages/docs/src/apis/StyleSheet/StyleSheet.stories.mdx

@@ -79,5 +79,5 @@ const styles = StyleSheet.create({
 
 ### hairlineWidth
 
-Enables borders of just one physical pixel on retina screens, otherwise it is
-equal to a CSS value of 1px.
+Equal to 1px. This is not implemented using screen density as browsers may
+round sub-pixel values down to `0`, causing the line not to be rendered.

packages/react-native-web/src/exports/StyleSheet/StyleSheet.js

@@ -57,6 +57,9 @@ const StyleSheet = {
     return result;
   },
   flatten: flattenStyle,
+
+  // `hairlineWidth` is not implemented using screen density as browsers may
+  // round sub-pixel values down to `0`, causing the line not to be rendered.
   hairlineWidth: 1
 };