Make it explicit that DoH requests are for IPv4 addresses only.

src/client.cpp

@@ -686,7 +686,7 @@ Client::State::resolve_tcp_doh( const std::string& host
         if (!e) return TcpLookup::create(TcpLookup::endpoint_type{move(addr), *portn_o}, host, port);
     }
 
-    auto rq_o = doh::build_request(host, ep);
+    auto rq_o = doh::build_request_ipv4(host, ep);  // TODO: IPv6
     if (!rq_o) return or_throw<TcpLookup>(yield, asio::error::invalid_argument);
 
     sys::error_code ec;

src/doh.cpp

@@ -109,8 +109,8 @@ dns_query(const std::string& name)
 }
 
 boost::optional<Request>
-build_request( const std::string& name
-             , const Endpoint& ep)
+build_request_ipv4( const std::string& name
+                  , const Endpoint& ep)
 {
     auto dq_o = dns_query(name);
     if (!dq_o) return boost::none;

src/doh.h

@@ -37,12 +37,12 @@ using Endpoint = std::string;
 // Return none if the base URL is invalid.
 boost::optional<Endpoint> endpoint_from_base(const std::string&);
 
-// Return a DoH request for the given `name`
+// Return a DoH request for IPv4 (type A) addresses of the given `name`,
 // to be sent to the given DoH `endpoint`.
 //
 // Return none if the name is invalid.
-boost::optional<Request> build_request( const std::string& name
-                                      , const Endpoint&);
+boost::optional<Request> build_request_ipv4( const std::string& name
+                                           , const Endpoint&);
 
 // Return the addresses in the answers for the given host
 // in the given response.