Improve APIDOC to properly generate sub-namespace docs

[jfpoilpret]

Currentl, doxygen generation seems to "forget" some namespaces (or more precisely, namespaces inside namespaces) in some of the generated files:

In the example above, as we can see, we have the following issues:

• the description of namespace devices is incorrect (that's the description for namespace devices::rtc)
• the following devices sub-namespaces are missing: audio, magneto, rf...)

The problem might be due to doxygen comments directly set before declarations like:

namespace devices::rtc 
{
...
}

[jfpoilpret]

Solution is to add doxygen documentation to not-compound namespace:

namespace aaa {
    /**
     * Doc...
     */
    namespace bbb {
         ...
    }
}