[firebase_messaging]: add additional documentation for the data in notifications

[ehhc]

Description

In the documentation of the firebase_messaging plugin could have a more detailed section about accessing the data send alongside the notification. It would help not to waste time debugging and finding the errors if the documentation would explain it a bit better..

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[ehhc]

I signed it!
:)

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[ehhc]

i am the only author of the change. Maybe the problem is that i pushed it using another computer than normal and therefore commited strange author informations..

[collinjackson]

Thanks for the contribution.

I'd like to consider making some breaking changes to allow more re-use of code across platforms, but I maybe for now documenting the current behavior is better than having it be confusing.

Can you please fix the analyzer failures?

[ehhc]

@collinjackson done