doc: update policy
doc: update policy

add note that prefix-set has either v4 or v6.

[skip ci]

Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@lab.ntt.co.jp>

a16/gobgp,ishidawataru/gobgp,osrg/gobgp,a16/gobgp,fujita/gobgp,tamihiro/gobgp,osrg/gobgp,a16/gobgp,Dipsingh/gobgp,ishidawataru/gobgp,a16/gobgp,a16/gobgp,fujita/gobgp,tamihiro/gobgp,fujita/gobgp,a16/gobgp,tamihiro/gobgp,j-kato/gobgp,osrg/gobgp,j-kato/gobgp,ishidawataru/gobgp,osrg/gobgp,j-kato/gobgp,a16/gobgp,tamihiro/gobgp,Dipsingh/gobgp,fujita/gobgp,Dipsingh/gobgp