docs: specify that FCM integration methods must be async

[mikehardy]

Causes problems noticed here if not async: invertase/react-native-notifee#14 (comment)

[mikehardy]

@Ehesp / @Salakar of interest - notifee problem discovered + solution provided => doc update

[Salakar]

Nice, thanks @mikehardy - I tweaked it slightly 😅

[mikehardy]

Yeah - I struggled with it honestly. In the type def for messaging in react-native-firebase repo they are actually not coherent! One of them (onMessage) makes no mention of needing a promise return or being async, but setBackgroundMessageHandler does. Not sure how to fix? The example is clear so perhaps just altering the typedef?

https://github.com/invertase/react-native-firebase/blob/f0959f2054d46578da7769d217cf02f793412a41/packages/messaging/lib/index.d.ts#L621-L647

[Salakar]

Have added it to this weeks planner to check out the types, so it should get looked at hopefully by someone 👍