How do you add custom help text to content entry/edit screens unrelated to field?
I usually do this with a form_alter myself in PHP code or I use the help entry for a Field.

Recently I found out about the Markup module. I recommend you try it out. I have no added it to any of my projects yet, but it's on my todo list.


  This module adds the ability for site admins to add "Markup" widgets
  to the form. These essentially let designers of cck content-types
  insert additional markup into the node/edit form to display to
  content-authors. This is the equivalent of adding additional elements
  to $form of the type '#markup'.
  
  The module does not add anything to the $node object for the content
  being created, and utilizes form_alter to remove unnecessary fields
  from the '_content_admin_field' form when this widget is being
  created.