Most elements in an XSD schema can contain an annotation. This is a human readable block that can be used to comment your schema.
When a graphical entity is selected in the XSD schema editor, any annotations for it are show in the documentation window.
If there are no annotations, then the documentation window will be empty. If you type your comment into the window then it will be added as an annotation to your selected entity.
If the AdditionalInfo Element was selected, then you would see the comment "This element is used to ...." in the documentation window.
<xs:documentation>This element is used to ....</xs:documentation>