Notes are better expressed with reST admonitions.

Fixes: 0ea8ce61 ("Documentation: LoongArch: Add basic documentations")
Reviewed-by: WANG Xuerui <git@xen0n.name>
Signed-off-by: Yanteng Si <siyanteng@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>