doc: Explain squashing with merge commits

CONTRIBUTING.md

@@ -216,9 +216,9 @@ Please update the resulting commit message, if needed. It should read as a
 coherent message. In most cases, this means not just listing the interim
 commits.
 
-If you have problems with squashing or other git workflows, you can enable
-"Allow edits from maintainers" in the right-hand sidebar of the GitHub web
-interface and ask for help in the pull request.
+If your change contains a merge commit, the above workflow may not work and you
+will need to remove the merge commit first. See the next section for details on
+how to rebase.
 
 Please refrain from creating several pull requests for the same change.
 Use the pull request that is already open (or was created earlier) to amend
@@ -231,7 +231,9 @@ pull request to pull request.
 ### Rebasing Changes
 
 When a pull request conflicts with the target branch, you may be asked to rebase it on top of the current target branch.
-The `git rebase` command will take care of rebuilding your commits on top of the new base.
+
+    git fetch https://github.com/bitcoin/bitcoin  # Fetch the latest upstream commit
+    git rebase FETCH_HEAD  # Rebuild commits on top of the new base
 
 This project aims to have a clean git history, where code changes are only made in non-merge commits. This simplifies
 auditability because merge commits can be assumed to not contain arbitrary code changes. Merge commits should be signed,