docs: rebaseWhen=conflicted + automerge (#9031)

docs/usage/configuration-options.md

@@ -1686,7 +1686,10 @@ Possible values and meanings:
 - `conflicted`: Renovate will rebase only if the branch is conflicted
 - `behind-base-branch`: Renovate will rebase whenever the branch falls 1 or more commit behind its base branch
 
-Note: this field replaces the previous fields of `rebaseConflictedPrs` and `rebaseStalePrs`.
+`rebaseWhen=conflicted` is not recommended if you have enabled Renovate automerge, because:
+
+- It could result in a broken base branch if two updates are merged one after another without testing the new versions together
+- If you have enforced that PRs must be up-to-date before merging (e.g. using branch protection on GitHub), then automerge won't be possible as soon as a PR gets out-of-date but remains non-conflicted
 
 ## recreateClosed