Add documentation FAQ entry about .nv cache directory

[seibert]

Based on this post:

https://groups.google.com/a/continuum.io/d/msg/numba-users/DcWYd7IR5HE/MtzPSIzcAQAJ

It sounds like there are situations where PTX that has been JIT compiled by the CUDA driver has been cached to $HOME/.nv (on Linux anyway) and can cause linker problems with Numba. We should add an entry about this to the FAQ, and possibly even detect this error and linux to the FAQ.

For more information about the CUDA JIT cache (not to be confused with the caching Numba does on the CPU), see this NVIDIA blog post:

https://devblogs.nvidia.com/cuda-pro-tip-understand-fat-binaries-jit-caching/