Update docs:

- Some comments in hip_hcc.cpp
 - document hipHostRegister* flags.
 - expand docs on hipHostRegister/Unregister. Use "register" rather than
   "pin" to describe action these take.
 - change required CUDA version to 6 (require unified memory)
 - remake doxygen.


[ROCm/hip commit: eced013ae4]

projects/hip/docs/markdown/hip_faq.md

@@ -68,7 +68,7 @@ HIP offers several benefits over OpenCL:
 
 ### What hardware does HIP support?
 - For AMD platforms, HIP runs on the same hardware that the HCC "hc" mode supports - specifically AMD Kaveri, Carrizo and Fiji.
-- For Nvidia platforms, HIP should run on a device which runs the CUDA SDK 5.0 or newer. We have tested the Nvidia Titan and K40.
+- For Nvidia platforms, HIP requires Unified Memory and should run on a device which runs the CUDA SDK 6.0 or newer. We have tested the Nvidia Titan and K40.
 
 ### Does Hipify automatically convert all source code?
 Typically, Hipify can automatically convert almost all run-time code, and the coordinate indexing device code.