I havent seen any microsoft source or any other dirty stuff but what I know is that the DirectX dlls call functions like ddentryxx and gdientryxx in gdi32.dll. (there is a ddrawgdi.h file in the platform SDK I have and there is also documentation on MSDN for them) These then call system calls that thunk down into kernel. In the kernel mode code, I believe that a combination of code in win32k.sys, code in dxapi.sys, code in dxg.sys and code in the display driver are what actually do the work. (although I dont know for sure the relationship or what is going on here)
Also, I dont know exactly how complete it is but I know that the windows DDK and/or the DirectX SDK includes header files like d3dnthal.h and that there is documentation for the stuff in those header files in MSDN.