Clarify JavaScript module naming edge case

Summary:
I've been burned by https://stackoverflow.com/questions/35052565/undefined-is-not-an-object-from-native-module-in-react-native/36580178 a few times, when I name a native module with RCT prefix. This will hopefully save me and others from that pain in the future.

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Closes #14903

Differential Revision: D5390343

Pulled By: hramos

fbshipit-source-id: 73667d703b097e70e15ff42c54b7ccc6b480104f

docs/NativeModulesIOS.md

@@ -29,7 +29,7 @@ A native module is just an Objective-C class that implements the `RCTBridgeModul
 @end
 ```
 
-In addition to implementing the `RCTBridgeModule` protocol, your class must also include the `RCT_EXPORT_MODULE()` macro. This takes an optional argument that specifies the name that the module will be accessible as in your JavaScript code (more on this later). If you do not specify a name, the JavaScript module name will match the Objective-C class name.
+In addition to implementing the `RCTBridgeModule` protocol, your class must also include the `RCT_EXPORT_MODULE()` macro. This takes an optional argument that specifies the name that the module will be accessible as in your JavaScript code (more on this later). If you do not specify a name, the JavaScript module name will match the Objective-C class name. If the Objective-C class name begins with RCT, the JavaScript module name will exclude the RCT prefix.
 
 ```objectivec
 // CalendarManager.m