It just isn't documented, there is already assigned a ticket at http://code.opencv.org/issues/1026, so there is no need to assign another one, however you can leave a note there that you actually use (or want to use) the descriptor.
If you want to speed up things you can read the original paper (Vincent Lepetit, Pascal Fua,"Towards Recognizing Feature points Using Classification Trees," Technical Report IC/2004/74, EPFL, 2004.), figure out how it works (/legacy/src/planardetect.cpp) and add the missing documentation to OpenCV. Please see http://code.opencv.org/projects/opencv/wiki/How_to_contribute how you can contribute.