Document the `Up` shortcut to "edit my last note" in the shortcuts help pane and shortcuts doc page

The awesome feature to edit your last note when hitting Up in an empty note textarea is currently undocumented!

The implementation is buried in https://gitlab.com/gitlab-org/gitlab-ce/blob/acfbeced52db321534c7b03a56a909e280d26914/app/assets/javascripts/notes.js.coffee#L105.

This should be documented in:

• The shortcuts help pane: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/views/help/_shortcuts.html.haml
• The shortcuts documentation image: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/workflow/shortcuts.png