docs: reasoning and use cases for channel upgradability

[charleenfei]

Description

closes: #5267

Commit Message / Changelog Entry

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[crodriguezvega]

Thank you for picking up a bunch of the documentation issues, @charleenfei.

[damiannolan]

Awesome! LGTM 🚀

Left a couple of nits and suggestions on wording, feel free to apply which ones you want :)

[DimitrisJim]

don't have much to add reading previous reviews, looks great to me! 🎊