[docs] fix the declarations of the llvm.nvvm.ptr.gen.to.* intrinsics

Summary:
These intrinsics should take a generic input address space and outputs a
non-generic address space.

Test Plan: no

Reviewers: jholewinski, eliben

Reviewed By: eliben

Subscribers: eliben, jholewinski, llvm-commits

Differential Revision: http://reviews.llvm.org/D10132

llvm-svn: 238620