It looks like this feature is available in Pega KM 7.4 and up.I was looking in 7.2 and didn't find it.
Marking article text as internal
Pega Knowledge authors can mark a sentence, paragraph, or entire section of an article as internal. When the content is marked as internal, the content is highlighted with a gray background that provides a visual indicator of internal content. Customer Service agents can see the full article. External users, such as KM help site users, cannot see the sections marked as internal.
To mark content as internal, perform the following steps:
1. In the left navigation panel, expand Knowledge and click Content.
2. Click the content that you want to mark as internal.
3. Click Edit.
4. To mark the content as internal, select the paragraph or text.
5. Click “internal use (employees only) padlock icon” in CKEDITOR ribbon