Clarify default MMQ for CUDA and LLAMA_CUDA_FORCE_MMQ flag

[isaac-mcfadyen]

• I have read the contributing guidelines
• Self-reported review complexity:
  • Low
  • Medium
  • High

Summary

• In CUDA: use MMQ instead of cuBLAS by default #8075, MMQ was enabled by default on GPUs with int8 tensor core support.
• A short description of the LLAMA_CUDA_FORCE_MMQ was added to the README. As it currently stands though, the message makes it seem like MMQ will not be used unless enabled with the flag.
• The message says "flag forces MMQ to be enabled on GPUs without int8 support" but doesn't really say that it will be enabled by default on GPUs with int8 support.
• This PR just adds a short blurb stating that MMQ is enabled by default on GPUs with int8 tensor core support, and that the flag forces it for all GPUs.