XML comments cannot be applied directly to a namespace within your code. As such, you must take an alternate approach to specifying namespace comments.
Your documentation build tool may provide support for defining project summary and namespace comments from within the development environment. For example, the Sandcastle Help File Builder provides a Summaries project property category that you can use to specify project summary and namespace summary comments. See your build tool's documentation for further information.
You can manage the XML comments for the project summary and namespace summaries in an external XML comments file.
Your documentation build tool may provide support for managing the namespace summary comments within your source code using a special hidden class called NamespaceDoc to which you attach the comments in each namespace.