[Doc]: documenting flash attention 1 vs 2 in env vars

[jamesbraza]

📚 The doc issue

Looking at https://docs.vllm.ai/en/latest/serving/env_vars.html for VLLM_ATTENTION_BACKEND:

• "FLASH_ATTN": use FlashAttention

This leaves me wondering if it's flash attention 1 or 2.

Suggest a potential alternative/fix

Appending this description with a 1 or 2, such as:

• "FLASH_ATTN": use FlashAttention-2

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[LucasWilkinson]

It will use FlashAttention v2 or v3 depending on device, this will likely expand with new versions of flash attention targeting newer hardware