[doc] fix doc build cmake flags (#682)
[doc] fix doc build cmake flags (#682)

OTBR_DBUS is required for generating the dbus server docs.
openthread/borderrouter,openthread/borderrouter,openthread/ot-br-posix,openthread/ot-br-posix,openthread/ot-br-posix,openthread/ot-br-posix,openthread/borderrouter,openthread/borderrouter,openthread/borderrouter,openthread/ot-br-posix,openthread/ot-br-posix,openthread/borderrouter