Move use of the "Submit merge request" button to the last step.  Add a
step for the "Title" field before the "Description" field.  In the
"Description" field, split various hints into bullet points to make them
more distinct.  Provide a complete example of a description.  Explicitly
state that the other fields may be left blank.

Also mention in the "Review a Merge Request" section that "@" syntax
presents completion.  It is better to use completion there than in the
Description field because it will complete developers with access to
the target repository instead of just the source repository.

Inspired-by: Ben Boeckel <ben.boeckel@kitware.com>