doc/contributing: Explicitly allow trivial changes by non-authors

Discussed and proposed at https://discourse.nixos.org/t/rules-around-adding-to-someones-pr/22671/3

Changed files
+6
doc
+6
doc/contributing/reviewing-contributions.chapter.md
···
It is possible for community members that have enough knowledge and experience on a special topic to contribute by merging pull requests.
+
In case the PR is stuck waiting for the original author to apply a trivial
+
change (a typo, capitalisation change, etc.) and the author allowed the members
+
to modify the PR, consider applying it yourself. (or commit the existing review
+
suggestion) You should pay extra attention to make sure the addition doesn't go
+
against the idea of the original PR and would not be opposed by the author.
+
<!--
The following paragraphs about how to deal with unactive contributors is just a proposition and should be modified to what the community agrees to be the right policy.