[Statepoints 4/4] Statepoint infrastructure for garbage collection: Documentation

This is the fourth and final patch in the statepoint series.  It contains the documentation for the statepoint intrinsics and their usage.

There's definitely still room to improve the documentation here, but I wanted to get this landed so it was available for others.  There will likely be a series of small cleanup changes over the next few weeks as we work to clarify and revise the documentation.  If you have comments or questions, please feel free to discuss them either in this commit thread, the original review thread, or on llvmdev.  Comments are more than welcome.

Reviewed by: atrick, ributzka
Differential Revision: http://reviews.llvm.org/D5683