Add comment to HostResolver::CreateStandaloneNetworkBoundResolver

Explicitly state that a non-nullopt value for
ManagerOptions::dns_config_overrides is not supported.

Bug: 1309094
Change-Id: Ic78a49a6d57e45398044bbf743db313cc08683e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3598858
Reviewed-by: Eric Orth <ericorth@chromium.org>
Commit-Queue: Stefano Duo <stefanoduo@google.com>
Cr-Commit-Position: refs/heads/main@{#995638}

net/dns/host_resolver.h

@@ -426,6 +426,8 @@ class NET_EXPORT HostResolver {
   // performed exclusively for `target_network`, lookups will fail if
   // `target_network` disconnects. This can only be used by network-bound
   // URLRequestContexts.
+  // Due to the current implementation, if `options` is specified, its
+  // DnsConfigOverrides parameter must be empty.
   // Only implemented for Android starting from Marshmallow.
   static std::unique_ptr<HostResolver> CreateStandaloneNetworkBoundResolver(
       NetLog* net_log,