This MR updates the section of the user's guide pertaining to X86 feature flags with the following changes:
the NCG backend now supports SIMD, so remove all text that says the contrary,
the LLVM backend does not "automatically detect" features. I don't know where this came from, but it's wrong in general.
For example, you will get different assembly whether you pass -mavx to GHC or not when using the LLVM backend.
