Document what to do if you're opening a PR in a repo with code owners

Author: samsimpson1

source/manual/merge-pr.html.md

@@ -84,3 +84,6 @@ If two developers worked on the same branch and individually contributed commits
 2. It's OK for someone other than the author to merge a PR, particularly if the author is not available. That person should be confident that the change doesn't have dependencies on other changes, and that it won't break `main`.
 3. If a PR is particularly good, remember to praise the author for it. Emoji are a great way of showing appreciation for a PR that fixes a problem you've been having, or implements something you've wanted to do for a while.
 4. It's sometimes OK for merges to happen when test suites are failing. This ability is limited to repository administrators and account owners, so ask them if you need them to force a merge. This is particularly useful in a catch-22 situation of two repositories with failing test suites that depend on each other.
+5. Some repositories have code owners set for either the entire repository or specific files/directories.
+This is so the people responsible for the repository have a chance to review changes made to important files.
+If your PR changes files that have a code owner, you should get a review as usual and one from a code owner.