Improve the Code review guidelines documentation

Signed-off-by: Rémy Coutable <remy@rymai.me>

CONTRIBUTING.md

@@ -485,24 +485,6 @@ Please ensure that your merge request meets the contribution acceptance criteria
 When having your code reviewed and when reviewing merge requests please take the
 [code review guidelines](doc/development/code_review.md) into account.
 
-### Getting your merge request reviewed, approved, and merged
-
-There are a few rules to get your merge request accepted:
-
-1. Your merge request should only be **merged by a [maintainer][team]**.
-  1. If your merge request includes only backend changes [^1], it must be
-    **approved by a [backend maintainer][team]**.
-  1. If your merge request includes only frontend changes [^1], it must be
-    **approved by a [frontend maintainer][team]**.
-  1. If your merge request includes frontend and backend changes [^1], it must
-    be **approved by a [frontend and a backend maintainer][team]**.
-1. To lower the amount of merge requests maintainers need to review, you can
-  ask or assign any [reviewers][team] for a first review.
-  1. If you need some guidance (e.g. it's your first merge request), feel free
-    to ask one of the [Merge request coaches][team].
-  1. The reviewer will assign the merge request to a maintainer once the
-    reviewer is satisfied with the state of the merge request.
-
 ### Contribution acceptance criteria
 
 1. The change is as small as possible

doc/development/code_review.md

 # Code Review Guidelines
 
+## Getting your merge request reviewed, approved, and merged
+
+There are a few rules to get your merge request accepted:
+
+1. Your merge request should only be **merged by a [maintainer][team]**.
+  1. If your merge request includes only backend changes [^1], it must be
+    **approved by a [backend maintainer][team]**.
+  1. If your merge request includes only frontend changes [^1], it must be
+    **approved by a [frontend maintainer][team]**.
+  1. If your merge request includes frontend and backend changes [^1], it must
+    be **approved by a [frontend and a backend maintainer][team]**.
+1. To lower the amount of merge requests maintainers need to review, you can
+  ask or assign any [reviewers][team] for a first review.
+  1. If you need some guidance (e.g. it's your first merge request), feel free
+    to ask one of the [Merge request coaches][team].
+  1. The reviewer will assign the merge request to a maintainer once the
+    reviewer is satisfied with the state of the merge request.
+
+## Best practices
+
 This guide contains advice and best practices for performing code review, and
 having your code reviewed.
 
@@ -12,7 +32,7 @@ of colleagues and contributors. However, the final decision to accept a merge
 request is up to one the project's maintainers, denoted on the
 [team page](https://about.gitlab.com/team).
 
-## Everyone
+### Everyone
 
 - Accept that many programming decisions are opinions. Discuss tradeoffs, which
   you prefer, and reach a resolution quickly.
@@ -31,8 +51,11 @@ request is up to one the project's maintainers, denoted on the
 - Consider one-on-one chats or video calls if there are too many "I didn't
   understand" or "Alternative solution:" comments. Post a follow-up comment
   summarizing one-on-one discussion.
+- If you ask a question to a specific person, always start the comment by
+  mentioning them; this will ensure they see it if their notification level is
+  set to "mentioned" and other people will understand they don't have to respond.
 
-## Having your code reviewed
+### Having your code reviewed
 
 Please keep in mind that code review is a process that can take multiple
 iterations, and reviewers may spot things later that they may not have seen the
@@ -50,11 +73,12 @@ first time.
 - Extract unrelated changes and refactorings into future merge requests/issues.
 - Seek to understand the reviewer's perspective.
 - Try to respond to every comment.
+- Let the reviewer select the "Resolve discussion" buttons.
 - Push commits based on earlier rounds of feedback as isolated commits to the
   branch. Do not squash until the branch is ready to merge. Reviewers should be
   able to read individual updates based on their earlier feedback.
 
-## Reviewing code
+### Reviewing code
 
 Understand why the change is necessary (fixes a bug, improves the user
 experience, refactors the existing code). Then:
@@ -69,12 +93,22 @@ experience, refactors the existing code). Then:
   someone else would be confused by it as well.
 - After a round of line notes, it can be helpful to post a summary note such as
   "LGTM :thumbsup:", or "Just a couple things to address."
+- Assign the merge request to the author if changes are required following your
+  review.
+- You should try to resolve merge conflicts yourself, using the [merge conflict
+  resolution][conflict-resolution] tool.
+- Set the milestone before merging a merge request.
 - Avoid accepting a merge request before the job succeeds. Of course, "Merge
   When Pipeline Succeeds" (MWPS) is fine.
 - If you set the MR to "Merge When Pipeline Succeeds", you should take over
   subsequent revisions for anything that would be spotted after that.
+- Consider using the [Squash and
+  merge][squash-and-merge] feature when the merge request has a lot of commits.
+
+[conflict-resolution]: https://docs.gitlab.com/ce/user/project/merge_requests/resolve_conflicts.html#merge-conflict-resolution
+[squash-and-merge]: https://docs.gitlab.com/ee/user/project/merge_requests/squash_and_merge.html#squash-and-merge
 
-## The right balance
+### The right balance
 
 One of the most difficult things during code review is finding the right
 balance in how deep the reviewer can interfere with the code created by a
@@ -100,7 +134,7 @@ reviewee.
   tomorrow. When you are not able to find the right balance, ask other people
   about their opinion.
 
-## Credits
+### Credits
 
 Largely based on the [thoughtbot code review guide].