Update llama_factory.rst which might be more user-friendly for beginners

[SuperAZHE]

I have a suggestion to enhance the script's description, which might be more user-friendly for beginners, reducing the level of confusion

[jklj077]

IMO, enabling fa2 by default is prone to problems. however, I agree that there should be section on how fa2 works in llama-factory that includes at least:

1. what kinds of devices are required
2. what improvements should be expected (if users are with pytorch like 2.3 and flash_attn=auto in llama_factory, there are chances that fa2 is already enabled through sdpa).

[jklj077]

ping @yangjianxin1 who is also working on this