doc: Explain that low-effort pull requests may be closed

CONTRIBUTING.md

@@ -78,6 +78,13 @@ The codebase is maintained using the "contributor workflow" where everyone
 without exception contributes patch proposals using "pull requests" (PRs). This
 facilitates social contribution, easy testing and peer review.
 
+Pull request authors must fully and confidently understand their own changes
+and must have tested them. Contributors should mention which tests cover their
+changes, or include the manual steps they used to confirm the change.
+Contributors are expected to be prepared to clearly motivate and explain their
+changes. If there is doubt, the pull request may be closed.
+Please refer to the [peer review](#peer-review) section below for more details.
+
 To contribute a patch, the workflow is as follows:
 
   1. Fork repository ([only for the first time](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/fork-a-repo))
@@ -338,6 +345,11 @@ reviewers that the changes warrant the review effort, and if reviewers are
 "Concept NACK'ing" the PR, the author may need to present arguments and/or do
 research backing their suggested changes.
 
+Moreover, if there is reasonable doubt that the pull request author does not
+fully understand the changes they are submitting themselves, or if it becomes
+clear that they have not tested the changes on a basic level themselves, the
+pull request may be closed immediately.
+
 #### Conceptual Review
 
 A review can be a conceptual review, where the reviewer leaves a comment