Add comment to clarify how I18nManager works

iOS I18nManager.isRTL is not wrong but often misinterpreted. Comment adds clarification to native call that Apple doesn't.

Author: scarlac

packages/react-native/React/Modules/RCTI18nUtil.m

@@ -95,7 +95,9 @@ - (BOOL)isDevicePreferredLanguageRTL
   return direction == NSLocaleLanguageDirectionRightToLeft;
 }
 
-// Check if the current application language is RTL
+// Intersect Xcode "Localization" languages with the system languages and checks if the result is using RTL
+// For RTL support, app must add an RTL lang to .pbxproj's knownRegions = ( ... )
+// and user must have chosen an RTL language
 - (BOOL)isApplicationPreferredLanguageRTL
 {
   NSWritingDirection direction = [NSParagraphStyle defaultWritingDirectionForLanguage:nil];