[ add ] style-guide recommendation preferring contradiction over ⊥-elim (#2798)

* add: recommendation

* fix: whitespace

* Update doc/style-guide.md

Improved text.

Co-authored-by: Jacques Carette <carette@mcmaster.ca>

* fix: `fill-column` to 72

---------

Co-authored-by: Jacques Carette <carette@mcmaster.ca>

Author: jamesmckinna

doc/style-guide.md

@@ -717,3 +717,8 @@ used successfully in PR
 systematic for `Nary` relations in PR
 [#811](https://github.com/agda/agda-stdlib/pull/811)
 
+## Prefer use of `Relation.Nullary.Negation.Core.contradiction`
+
+Where possible use `contradiction` between two explicit arguments rather
+than appealing to the lower-level `Data.Empty.⊥-elim`. This provides
+clearer documentation for readers of the code.