G-API: Documentation updates

G-API: Documentation updates

1) Document GFrame/MediaFrame (and also other G-API types)
- Added doxygen comments for GMat, GScalar, GArray<T>, GOpaque classes;
- Documented GFrame and its host-side counterpart MediaFrame;
- Added some more notes to the data type classes.

2) Give @brief descriptions to most of the cv::gapi::* namespaces

3) Make some symbols private
- These structures are mainly internal and shouldn't be used directly