doc: add text about moving long commit lists out of PR description

Author: danielleadams

doc/guides/releases.md

@@ -398,7 +398,19 @@ Create a pull request targeting the correct release line. For example, a
 `v5.3.0-proposal` PR should target `v5.x`, not master. Paste the CHANGELOG
 modifications into the body of the PR so that collaborators can see what is
 changing. These PRs should be left open for at least 24 hours, and can be
-updated as new commits land.
+updated as new commits land. If the CHANGELOG is long enough that it slows
+down the GitHub UI, consider pasting the commits into `<details>` tags or
+in follow up comments.
+
+If using the `<details>` tag, use the following format:
+
+```markdown
+<details>
+<summary>Commits</summary>
+
+* Full list of commits...
+</details>
+```
 
 If you need any additional information about any of the commits, this PR is a
 good place to @-mention the relevant contributors.