Use GetDesireableServiceFlags in static seeds, document this.

44407100f broke inserting entries into addrman from static seeds
(as well as dnsseeds which did not support service bits). Static
seeds were already being filtered by UA for 0.13.1+ (ie
NODE_WITNESS), so simply changing the default service bits to
include NODE_WITNESS (and updating docs appropriately) is
sufficient.

For DNS Seeds, we will later fix by falling back to oneshot if a
seed does not support filtering.

src/net.cpp

@@ -136,7 +136,7 @@ static std::vector<CAddress> convertSeed6(const std::vector<SeedSpec6> &vSeedsIn
     for (const auto& seed_in : vSeedsIn) {
         struct in6_addr ip;
         memcpy(&ip, seed_in.addr, sizeof(ip));
-        CAddress addr(CService(ip, seed_in.port), NODE_NETWORK);
+        CAddress addr(CService(ip, seed_in.port), GetDesirableServiceFlags(NODE_NONE));
         addr.nTime = GetTime() - GetRand(nOneWeek) - nOneWeek;
         vSeedsOut.push_back(addr);
     }