Improve unboxed sum documentation

* Clarify the representation of sums without fields.
* Try to improve language, clarity, and examples.

Fixes #14752

Reviewers: osa1, bgamari

Reviewed By: osa1

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #14752

Differential Revision: https://phabricator.haskell.org/D4379

(cherry picked from commit d987f71a)