clarify wording of GPU driver-related notes

After getting the ill-documented return value of cudaDriverGetVersion
clarified with NVIDIA driver engineers, the notes and warnings have been
clarified and now refer to driver API version 5.0.

Change-Id: I66cda563cd6753e5337ac0ed28264a3941627a03
Merge "clarify wording of GPU driver-related notes" into release-4-6