We need to add to our Sharepoint screen an explanation of related sources, not in a document but directly above the file listing. I have seen headings and text blocks on Sharepoint screens but cannot find an explanation of how to add them.
Sounds like you want to modify the "Description" box on things like Columns, Lists and Sites.
This will display on the top of list views, and next to fields on list edit forms.
Additionally: You said "above the file listing", so this suggests you want it shown on a Document Library, go into Library Settings, click "Title and Description" and you'll be able to change the description text.
First, ensure that you are at least a Contributor on the site collection you wish to edit. If so, navigate to the site and click [Edit] in the top right hand corner. If you click into the rectangle that represents a part of the body of the page, you should be able to enter text.
Remember to check in your changes. :)
Edit: After reading your post again, can you clarify if you are just wanting to add text to page, or add text to the view of a document library?