Allow documenting the module specific namespace docs

Only one location can use the \namespace command, otherwise it gets overwritten. If you document items under that namespace in other modules, and you have the dependency to the index that contains the \namespace, you should get the namespacename-sub-modulename.html page.

Would be nice if it was possible to document the sub-page as well, for module specific docs about the namespace, eg:
 

\namespace Foo
\inmodule SomeSubModule

 
Where the main docs are documented with a plain:

\namespace Foo