As far as I understand your question about creating documentation for external use of the DLL, I see two possible ways you can go:
- Add “missing” notes for all elements of the external DLL that you can document or
- Add conceptual themes to your own program documentation.
In my test solution, the WindowsApplication2 project has the form for adding two values using a simple PDUNZDLL. The Sandcastle Builder project "Documentation1" and two documentation sources were added (at this point, without an XML comment file, see the first screenshot below). You know, a DLL without an XML comment file leads to a red missing summary.

Proposed Solution (1):
Create an empty XML comment file as follows and name it after assembly with the extension .xml, for example. PDUNZDLL.xml
<?xml version="1.0"?> <doc> <assembly> <name>PDUNZDLL</name> </assembly> <members> </members> </doc>
Save this file to the Debug folder D:\Visual-Studio-2015\Projects\WindowsApplication2\WindowsApplication2\bin\Debug
- Double-click "Project Properties" (see second screenshot below)
- In the Component Configurations dialog box, add the IntelliSense component to the project.
- Select Missing Tags and set the Show Missing Tags property to your liking. This will force the assembly to add “missing” notes for all the elements you want to document.
- Create a project and you will find a new XML comment file named after the assembly in the project output folder, for example.
D:\Visual-Studio-2015\Projects\WindowsApplication2\Documentation1\Help - Edit the
<member> elements in the XML comment file to add comments to each member, as shown in the second snapshot.
When you're done, replace the original placeholder file, for example. D:\Visual-Studio-2015\Projects\WindowsApplication2\WindowsApplication2\bin\Debug\PDUNZDLL.xml with the one that was created from the assembly with your edited comments. Reinstall the draft documentation.
This, of course, takes a lot of time, like any help in building applications. And actually it was the task of a DLL developer.

Proposed Solution (2):
Reading between your lines "... so that others can understand why I make certain calls that I make for these external DLL files." leads me to the proposal to add additional documentation to my own program.
So, I mean adding conceptual topics that describe what you call the functions of external DLL files.
I deleted the DLL in the "Sources of documentation" section, added a new folder "ExternalDLL", added a new item "Conceptual and walkthrough", double-click ContentLayout.content in Solution explorer, and completed all the steps to compose the content, ReBuild documentation project. resulting in a help file as shown in the third snapshot below (see also help information).

Congratulations on your help!
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