Test and Document Exception.__cause__ on NetworkError

[Bibo-Joshi]

Closes #3778 as suggested by Noam in the dev chat.

Tests for {Base, HTTPX}Request not extended yet.

[lemontree210]

I have a general question: it was my understanding that if we raise some exception from another exception, we don't need to specify any cause explicitly. I think that's the way it's formulated in the docs and I personally have found raise ... from ... quite informative on its own.

If a user wants to check the cause explicitly, I think they can use .__cause__ (as per the same docs).

So what am I missing here? :)

[Bibo-Joshi]

Huh. I really just didn't know that. Makes sense that raise … from has to be stored somehow, but I wasn't aware that this is documented. A bit weird though that __cause__ is not documented as proper attribute 🤔 Moreorever, that attribute apparently is not pickled: python/cpython#73652.
I'd be okay with the non-pickable thing if we agree that __cause__ can bes considered to be public python API …

[lemontree210]

Some punctuation nitpicks in docstring, maybe not even justified :)