Clearer information regarding RCT_EXPORT_MODULE

Summary:
I think this update is Critical as I wanted to add a different module name to be the same as the Android module name. I simply lost 6 hours until I found out that the Macro does not accept String syntax. Almost drove me crazy. Please, at least do something about it. Thanks.

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Closes https://github.com/facebook/react-native/pull/11634

Differential Revision: D4533312

Pulled By: lacker

fbshipit-source-id: 2c547f791ed76ba62e0c7e5cabc2830dbae34075

docs/NativeModulesIOS.md

@@ -34,8 +34,12 @@ In addition to implementing the `RCTBridgeModule` protocol, your class must also
 // CalendarManager.m
 @implementation CalendarManager
 
+// To export a module named CalendarManager
 RCT_EXPORT_MODULE();
 
+// This would name the module AwesomeCalendarManager instead
+// RCT_EXPORT_MODULE(AwesomeCalendarManager);
+
 @end
 ```