Grammar and wording improvements

Improve the grammar and wording of the fast-forward merge and
rebase-before-merge documentation.

[ci skip]

doc/workflow/ff_merge.md

 # Fast-forward merge
 
-GitLab Enterprise Edition offers a way to accept merge request without creating merge commit.
-If you prefer linear git history - this might be a good feature for you.
-You can configure this per project basis by navigating to the project settings page and selecting `Only fast-forward merging` checkbox.
+GitLab Enterprise Edition offers a way to accept merge requests without creating merge commits.
+If you prefer a linear git history, this might be a good feature for you.
+You can configure this on a per-project basis by navigating to the project settings page and selecting the **Only fast-forward merging** checkbox.
 
 ![Merge request settings](ff_merge.png)
 
-Now when you visit merge request page you will be able to accept it only if fast-forward merge is possible. 
-If target branch is ahead of source branch - you need to rebase source branch before you will be able to do fast-forward merge.
+Now when you visit the merge request page, you will be able to accept it only if a fast-forward merge is possible.
+If the target branch is ahead of the source branch, you need to rebase the source branch before you will be able to do a fast-forward merge.
 
-For simple rebase operations you can use [Rebase before merge](rebase_before_merge.md) feature. 
+For simple rebase operations, you can use the [Rebase before merge](rebase_before_merge.md) feature.

doc/workflow/rebase_before_merge.md

 # Rebase before merge
 
-GitLab Enterprise Edition offers a way to rebase source branch of merge request. 
-This feature is part of [Fast-forward merge](ff_merge.md) feature. 
-It allows you to rebase source branch of merge request in order to perform fast-forward merge.
+GitLab Enterprise Edition offers a way to rebase the source branch of a merge request.
+This feature is part of the [Fast-forward merge](ff_merge.md) feature.
+It allows you to rebase the source branch of a merge request in order to perform a fast-forward merge.
 
-You can configure this per project basis by navigating to the project settings page and selecting `Rebase button` checkbox.
-This checkbox is visible only if you have `Only fast-forward merging` checkbox enabled.
+You can configure this on a per-project basis by navigating to the project settings page and selecting the **Rebase button** checkbox.
+This checkbox is visible only if you have the **Only fast-forward merging** checkbox enabled.
 
 ![Merge request settings](merge_request_settings.png)
 
 
-Now if fast-forward merge requires rebase - you will see rebase button:
+Now, if a fast-forward merge requires a rebase, you will see a button to perform a rebase:
 
 ![Rebase request widget](rebase_request_widget.png)
 
-GitLab will attempt to rebase source branch. If rebase succeed you will see `Accept merge request` button.
-If clean rebase is not possible - you need to do rebase manually. 
-Possibly rebase requires some conflicts to be resolved by human.
+GitLab will attempt to rebase the source branch. If the rebase succeeds you will see an **Accept merge request** button.
+If a clean rebase is not possible, you will need to rebase manually.
+Rebasing may require some conflicts to be resolved manually.