doc/release-notes: tweaks (#315520)

* Clean up the curious dwarf-fortress note

(cherry picked from commit e36f83ea)

* Various tweaks to release notes

(cherry picked from commit 9708aca8)

* Consistently use capitalized Nix in plain text

(cherry picked from commit 005ef76e)

* Further tweaks to release notes

(cherry picked from commit 9c686124)

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Co-authored-by: Jacek Generowicz <jacg@my-post-office.net>