docs(firebase_auth): Removed duplicates; fixed typos; removed "unnecessary use of a null check"

[Nikita0x]

Description

Docs:
Fixing typos, removing duplicates

Code:
Removed the "unnecessary use of a null check". Actually, it wasn't me but a compiler forced me to do it!

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[Nikita0x]

I am not sure why some tests failed to be honest. I would like a help from somebody, I just changed the text in the docs mostly