Improve details about when rebase button is offered

doc/user/project/merge_requests/fast_forward_merge.md

@@ -15,7 +15,7 @@ to accept merge requests without creating merge commits.
 When the fast-forward merge
 ([`--ff-only`](https://git-scm.com/docs/git-merge#git-merge---ff-only)) setting
 is enabled, no merge commits will be created and all merges are fast-forwarded,
-which means that merging is only allowed if the branch could be fast-forwarded.
+which means that merging is only allowed if the branch can be fast-forwarded.
 
 When a fast-forward merge is not possible, the user is given the option to rebase.
 
@@ -28,9 +28,15 @@ When a fast-forward merge is not possible, the user is given the option to rebas
 Now, when you visit the merge request page, you will be able to accept it
 **only if a fast-forward merge is possible**.
 
+![Fast forward merge request](img/ff_merge_mr.png)
+
+If a fast-forward merge is not possible but a conflict free rebase is possible,
+a rebase button will be offered.
+
 ![Fast forward merge request](img/ff_merge_rebase.png)
 
-If the target branch is ahead of the source branch, you need to rebase the
+If the target branch is ahead of the source branch and a conflict free rebase is
+not possible, you need to rebase the
 source branch locally before you will be able to do a fast-forward merge.
 
 ![Fast forward merge rebase locally](img/ff_merge_rebase_locally.png)