[Docs] improve the LoRA doc. (#4838)
* improve the LoRA doc.
* include fuse_lora and unfuse_lora
* Apply suggestions from code review
Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>
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Co-authored-by: Steven Liu <59462357+stevhliu@users.noreply.github.com>
