doc: improve bmp documentation
doc: improve bmp documentation

Add documentation for route-monitoring-policy = "both".

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