[doxyrest] handle id collisions more intelligently
[doxyrest] handle id collisions more intelligently

turns out, id collisions are to be expected (e.g. libusb defines
documentation in .c files, while .h files have declaratios only). the
logic is: prefer the new one if and only if the old one had no
documentations.

vovkos/doxyrest,vovkos/doxyrest,vovkos/doxyrest,vovkos/doxyrest,vovkos/doxyrest