clarify contraction behavior and -cl-mad-enable

OpenCL contraction behavior is complicated, especially the interaction between `#pragma OPENCL FP_CONTRACT` and the `-cl-mad-enable` build option, and the subtle difference between an `fma` and a `mad`. We should try to clarify the contraction behavior, perhaps via some informative notes, so it is a little less confusing.

No source file, test, or entry point is identified. Start by locating the documentation for #pragma OPENCL FP_CONTRACT and -cl-mad-enable, then compare how the project currently explains fma versus mad. Done means adding a concrete explanation of their interaction and the resulting contraction behavior.

Written by the indexing model from the issue text.