RAJAPerf concerns

[MrBurmark]

We mentioned a few concerns at the meeting today.

• Make it more clear in the build section which versions of RAJA/camp/etc. go with what version of RAJAPerf. Probably mention our versioning scheme.
• Update the doc to the relevant RAJAPerf patch version.
• Differentiate between which versions of things can change (like the rocm/cuda/... version) and those that should stay the same.
• Make it clear that they must use consistent versions of RAJA and RAJAPerf and that its best to keep up to date with RAJAPerf releases. They should want to do this anyway as RAJA should improve over time.

[rhornung67]

All of these concerns will be addressed in the final version of the benchmarking docs.