Update receiving-messages.md

[VilleMiekkoja]

Improve docs; Request permissions section only relevant for iOS. On android, it always resolves, and has no use or effect on anything.

[mikehardy]

I also fear it may have changed with Android 10 - I'm not sure. Have you consulted the docs on react-native-permissions? Looks like it may be possible for Notifications to be disabled in Android under some cases:

https://github.com/react-native-community/react-native-permissions/blob/master/android/src/main/java/com/reactnativecommunity/rnpermissions/RNPermissionsModule.java#L89

So even though in v5 of react-native-firebase it always resolves to true, in reality now (after v5 here was written, and is now orphaned) it is a good idea to check.

For that reason I suggested adding a pointer to the 'react-native-permissions' library and mentioning it should be used prior to calling requestPermissions here, to make sure you really are handling permissions correctly

[gianpaj]

I strongly suggest adding this with Mike's addition. I was getting an FCM push token (somehow) but the app was still not registering for Push Notifications with iOS (no Notifications access under the Settings -> App).

After adding react-native-permissions and its requestNotifications() method the app finally prompted of the access 🎉

[CLAassistant]

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