docs(messaging): note headless task key change in release notes

[adamfellon]

When converting to v6 the key used when registering a headless task needs to be changes to in order to register the correct task.

Summary

Want to represent this comment for users who encounter headless task issues: #2412 (comment)

Checklist

• Supports Android

[CLAassistant]

All committers have signed the CLA.

[mikehardy]

CI seems to have had a major, unrelated failure @Salakar ?

[Salakar]

CI seems to have had a major, unrelated failure @Salakar ?

Its just incorrectly not ignoring the docs branch, have updated to branch rules so shouldn't do it again.

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Generally ok with this change but really AppRegistry.registerHeadlessTask shouldn't be called manually on v6. This is automatically done for you if you use messaging().setBackgroundMessageHandler(fn) - right now this is all it does internally - but moving it into the API gives us the flexibility in future to change it or further augment the API with minimal effort for users + it also matches the Firebase Web SDK API.

Would you be able to update this to mention calling that method instead? As with the headless task API; it should be called early on in your apps code.

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[adamfellon]

Ah, yeah I think I should even be calling that function in my code as well as it'll avoid any issues with renaming in the future.

I don't use Typescript so I think I missed the example from the interface.

I don't know how constants/statics from the NativeModules work but is it worth it to go as far as exporting private static final String TASK_KEY = "ReactNativeFirebaseMessagingHeadlessTask"; out to be used in lib/index.js when it registers the headless task?

[Salakar]

Superseded by #3339 and friends - thanks!

[Salakar]

Closing as #3339 which supersedes this has now been merged and is available on an RC release (6.4.0-rc4).

Thank you for your contribution