Explain usage of `<T: Config>` in FRAME storage + Update parachain pallet template (#4941)

Explains one of the annoying parts of FRAME storage that we have seen
multiple times in PBA everyone gets stuck on.

I have not updated the other two templates for now, and only reflected
it in the parachain template. That can happen in a follow-up.

- [x] Update possible answers in SE about the same topic.

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Co-authored-by: Serban Iorga <serban@parity.io>
Co-authored-by: command-bot <>