The NamespaceDoc class can be used to document namespace.
Is a GroupDoc class exist also to ducment the Namespace Group.
For example on your documentation
SandcastleBuilder.Components is a group and also a namespace.
"This namespace contains BuildAssembler related namespaces." is the comment for the group
"This namespace contains custom build components used by the Sandcastle BuildAssembler tool. The components are used to add additional features to the help topics such as a code colorization, alternate framework data caching implementations, etc."
is the comment for the namespace.