Merge bitcoin#26040: doc: comment "add only reachable addresses to addrman"

ce42570 doc: comment "add only reachable addresses to addrman" (Kristaps Kaupe)

Pull request description:

  Proposed by Sjors during review of bitcoin#25678, was likely just missed, as it also for me looks a code where comment will not hurt.

  bitcoin#25678 (comment)

ACKs for top commit:
  mzumsande:
    ACK ce42570
  vasild:
    ACK ce42570
  Zero-1729:
    re-ACK ce42570

Tree-SHA512: ef085d527349de07c1b43ed39e55e34b29cb0137c9509bd14a1af88206f7d4aa7dfec1dca53a9deaed67a2d0f32fa21e0b1a04d4d5d7f8a265dfab3b62bf8c54

Author: MacroFake

src/net.cpp

@@ -1648,6 +1648,14 @@ void CConnman::ThreadOpenConnections(const std::vector<std::string> connect)
 
             if (add_fixed_seeds_now) {
                 std::vector<CAddress> seed_addrs{ConvertSeeds(Params().FixedSeeds())};
+                // We will not make outgoing connections to peers that are unreachable
+                // (e.g. because of -onlynet configuration).
+                // Therefore, we do not add them to addrman in the first place.
+                // Note that if you change -onlynet setting from one network to another,
+                // peers.dat will contain only peers of unreachable networks and
+                // manual intervention will be needed (either delete peers.dat after
+                // configuration change or manually add some reachable peer using addnode),
+                // see <https://github.com/bitcoin/bitcoin/issues/26035> for details.
                 seed_addrs.erase(std::remove_if(seed_addrs.begin(), seed_addrs.end(),
                                                [](const CAddress& addr) { return !IsReachable(addr); }),
                                 seed_addrs.end());