Merge pull request #383 from microsoft/dev/drnoakes/doc-capabilites-from-dtb

Doc that capabilities cannot come from DTB

Author: drewnoakes

doc/overview/about_project_capabilities.md

@@ -43,6 +43,9 @@ being to add this MSBuild item to your `.targets` file:
 </ItemGroup>
 ```
 
+> [!IMPORTANT]
+> Project capabilites cannot be added during design-time builds. They must be added via evaluation, or via code.
+
 ## Defining fixed capabilities for a project type
 
 Some capabilities are static/fixed for a given project type. These capabilities should be defined directly on the project type registration.