Tweak documentation and update changelog.md

This adds release note entries to changelog and tweaks Haddock comments
with respect to the recent commits 1a9abe7a1a3c7 (re #8797) and
f932b79948f0 (re #8745)

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
Tweak Haddock comments in `Data.Ord.Down` added in ebc85262c

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>