Update docs for new commit changes modal
The introduction of an updated delete/edit commit modal requires new and updated documentation.
Required docs:
- New content to edit a file in the Web Editor from a merge request should be added.
- For edit while collaboration on merge requests across forks, this info should be added: Users should see the following message: "Your changes can be committed to %{branchName} because a merge request is open".
- When another user has already edited a file, this error message should be documented: "Someone edited the file the same time you did. Please check out the file and make sure your change will not unintentionally remove theirs".
- Information that selecting the cancel button pops up a confirmation modal.