add comments to describe the dns insert behavior
add comments to describe the dns insert behavior

Signed-off-by: Wang Xu <0e9dfdf30d71e7ded2bba4a3dc006039c4b00f3d@gmail.com>

gnawux/hyper,laijs/hyper,gnawux/hyper,laijs/hyper,gnawux/hyper,feiskyer/hyper,ZJU-SEL/hyper,laijs/hyperd,laijs/hyper,laijs/hyper,gao-feng/hyper,hyperhq/hyper,bergwolf/hyperd,ZJU-SEL/hyper,ZJU-SEL/hyper,bergwolf/hyperd,gao-feng/hyper-public,gao-feng/hyper-public,hyperhq/hyper,feiskyer/hyper,laijs/hyperd,hyperhq/hyperd,bergwolf/hyperd,gao-feng/hyper,feiskyer/hyper,hyperhq/hyper,laijs/hyperd,hyperhq/hyperd,gao-feng/hyper,hyperhq/hyperd,bergwolf/hyperd,ZJU-SEL/hyper,gao-feng/hyper-public,feiskyer/hyper,gao-feng/hyper,hyperhq/hyper